Fix tests / code to work with frozen string literals

author: Paul Kmiec <[email protected]> 2023-05-04 16:31:08 -0700
committer: Paul Kmiec <[email protected]> 2023-05-04 18:02:55 -0700
commit: fabe8cb2571c8865270247ca78ddc01d493a9ee1 (patch)
tree: 27fe0b0df8c9bec8f746d92a0d128cad197027b2 /lib/axlsx
parent: fef93ec8ae2caf8a3f8310dbf8101c103e5905e4 (diff)
download: caxlsx-fabe8cb2571c8865270247ca78ddc01d493a9ee1.tar.gz
caxlsx-fabe8cb2571c8865270247ca78ddc01d493a9ee1.zip
128 files changed, 323 insertions, 324 deletions
diff --git a/lib/axlsx/content_type/abstract_content_type.rb b/lib/axlsx/content_type/abstract_content_type.rb
index dfb7dd91..58ca8b2c 100644
--- a/lib/axlsx/content_type/abstract_content_type.rb
+++ b/lib/axlsx/content_type/abstract_content_type.rb
@@ -22,7 +22,7 @@ module Axlsx
     alias :ContentType= :content_type=
 
     # Serialize the contenty type to xml
-    def to_xml_string(node_name = '', str = '')
+    def to_xml_string(node_name = '', str = +'')
       str << "<#{node_name} "
       str << Axlsx.instance_values_for(self).map { |key, value| Axlsx::camel(key) << '="' << value.to_s << '"' }.join(' ')
       str << '/>'
diff --git a/lib/axlsx/content_type/content_type.rb b/lib/axlsx/content_type/content_type.rb
index ead50713..15751b17 100644
--- a/lib/axlsx/content_type/content_type.rb
+++ b/lib/axlsx/content_type/content_type.rb
@@ -14,9 +14,9 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<?xml version="1.0" encoding="UTF-8"?>'
-      str << ('<Types xmlns="' << XML_NS_T << '">')
+      str << (+'<Types xmlns="' << XML_NS_T << '">')
       each { |type| type.to_xml_string(str) }
       str << '</Types>'
     end
diff --git a/lib/axlsx/content_type/default.rb b/lib/axlsx/content_type/default.rb
index ad215070..16189fc6 100644
--- a/lib/axlsx/content_type/default.rb
+++ b/lib/axlsx/content_type/default.rb
@@ -16,7 +16,7 @@ module Axlsx
     alias :Extension= :extension=
 
     # Serializes this object to xml
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(NODE_NAME, str)
     end
   end
diff --git a/lib/axlsx/content_type/override.rb b/lib/axlsx/content_type/override.rb
index fd98a803..be45db3c 100644
--- a/lib/axlsx/content_type/override.rb
+++ b/lib/axlsx/content_type/override.rb
@@ -16,7 +16,7 @@ module Axlsx
     alias :PartName= :part_name=
 
     # Serializes this object to xml
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(NODE_NAME, str)
     end
   end
diff --git a/lib/axlsx/doc_props/app.rb b/lib/axlsx/doc_props/app.rb
index fec0a899..3eae1e28 100644
--- a/lib/axlsx/doc_props/app.rb
+++ b/lib/axlsx/doc_props/app.rb
@@ -219,9 +219,9 @@ module Axlsx
 
     # Serialize the app.xml document
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<?xml version="1.0" encoding="UTF-8"?>'
-      str << ('<Properties xmlns="' << APP_NS << '" xmlns:vt="' << APP_NS_VT << '">')
+      str << (+'<Properties xmlns="' << APP_NS << '" xmlns:vt="' << APP_NS_VT << '">')
       Axlsx.instance_values_for(self).each do |key, value|
         node_name = Axlsx.camel(key)
         str << "<#{node_name}>#{value}</#{node_name}>"
diff --git a/lib/axlsx/doc_props/core.rb b/lib/axlsx/doc_props/core.rb
index e7e53246..8d3292e9 100644
--- a/lib/axlsx/doc_props/core.rb
+++ b/lib/axlsx/doc_props/core.rb
@@ -22,13 +22,13 @@ module Axlsx
 
     # serializes the core.xml document
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<?xml version="1.0" encoding="UTF-8"?>'
-      str << ('<cp:coreProperties xmlns:cp="' << CORE_NS << '" xmlns:dc="' << CORE_NS_DC << '" ')
-      str << ('xmlns:dcmitype="' << CORE_NS_DCMIT << '" xmlns:dcterms="' << CORE_NS_DCT << '" ')
-      str << ('xmlns:xsi="' << CORE_NS_XSI << '">')
-      str << ('<dc:creator>' << self.creator << '</dc:creator>')
-      str << ('<dcterms:created xsi:type="dcterms:W3CDTF">' << (created || Time.now).strftime('%Y-%m-%dT%H:%M:%S') << 'Z</dcterms:created>')
+      str << (+'<cp:coreProperties xmlns:cp="' << CORE_NS << '" xmlns:dc="' << CORE_NS_DC << '" ')
+      str << (+'xmlns:dcmitype="' << CORE_NS_DCMIT << '" xmlns:dcterms="' << CORE_NS_DCT << '" ')
+      str << (+'xmlns:xsi="' << CORE_NS_XSI << '">')
+      str << (+'<dc:creator>' << self.creator << '</dc:creator>')
+      str << (+'<dcterms:created xsi:type="dcterms:W3CDTF">' << (created || Time.now).strftime('%Y-%m-%dT%H:%M:%S') << 'Z</dcterms:created>')
       str << '<cp:revision>0</cp:revision>'
       str << '</cp:coreProperties>'
     end
diff --git a/lib/axlsx/drawing/area_chart.rb b/lib/axlsx/drawing/area_chart.rb
index 9b948808..8339c255 100644
--- a/lib/axlsx/drawing/area_chart.rb
+++ b/lib/axlsx/drawing/area_chart.rb
@@ -74,16 +74,16 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
-        str << ("<c:" << node_name << ">")
-        str << ('<c:grouping val="' << grouping.to_s << '"/>')
-        str << ('<c:varyColors val="' << vary_colors.to_s << '"/>')
+        str << (+"<c:" << node_name << ">")
+        str << (+'<c:grouping val="' << grouping.to_s << '"/>')
+        str << (+'<c:varyColors val="' << vary_colors.to_s << '"/>')
         @series.each { |ser| ser.to_xml_string(str) }
         @d_lbls.to_xml_string(str) if @d_lbls
         yield if block_given?
         axes.to_xml_string(str, :ids => true)
-        str << ("</c:" << node_name << ">")
+        str << (+"</c:" << node_name << ">")
         axes.to_xml_string(str)
       end
     end
diff --git a/lib/axlsx/drawing/area_series.rb b/lib/axlsx/drawing/area_series.rb
index 55c391d9..9d87d078 100644
--- a/lib/axlsx/drawing/area_series.rb
+++ b/lib/axlsx/drawing/area_series.rb
@@ -71,15 +71,15 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
         if color
           str << '<c:spPr><a:solidFill>'
-          str << ('<a:srgbClr val="' << color << '"/>')
+          str << (+'<a:srgbClr val="' << color << '"/>')
           str << '</a:solidFill>'
           str << '<a:ln w="28800">'
           str << '<a:solidFill>'
-          str << ('<a:srgbClr val="' << color << '"/>')
+          str << (+'<a:srgbClr val="' << color << '"/>')
           str << '</a:solidFill>'
           str << '</a:ln>'
           str << '<a:round/>'
@@ -94,7 +94,7 @@ module Axlsx
 
         @labels.to_xml_string(str) unless @labels.nil?
         @data.to_xml_string(str) unless @data.nil?
-        str << ('<c:smooth val="' << ((smooth) ? '1' : '0') << '"/>')
+        str << (+'<c:smooth val="' << ((smooth) ? '1' : '0') << '"/>')
       end
     end
 
diff --git a/lib/axlsx/drawing/axes.rb b/lib/axlsx/drawing/axes.rb
index c84b4378..0baad85f 100644
--- a/lib/axlsx/drawing/axes.rb
+++ b/lib/axlsx/drawing/axes.rb
@@ -29,11 +29,11 @@ module Axlsx
     # @option options ids
     # If the ids option is specified only the axis identifier is
     # serialized. Otherwise, each axis is serialized in full.
-    def to_xml_string(str = '', options = {})
+    def to_xml_string(str = +'', options = {})
       if options[:ids]
         # CatAxis must come first in the XML (for Microsoft Excel at least)
         sorted = axes.sort_by { |name, axis| axis.kind_of?(CatAxis) ? 0 : 1 }
-        sorted.each { |axis| str << ('<c:axId val="' << axis[1].id.to_s << '"/>') }
+        sorted.each { |axis| str << (+'<c:axId val="' << axis[1].id.to_s << '"/>') }
       else
         axes.each { |axis| axis[1].to_xml_string(str) }
       end
diff --git a/lib/axlsx/drawing/axis.rb b/lib/axlsx/drawing/axis.rb
index 6ee2b9f4..38e57da5 100644
--- a/lib/axlsx/drawing/axis.rb
+++ b/lib/axlsx/drawing/axis.rb
@@ -148,11 +148,11 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
-      str << ('<c:axId val="' << @id.to_s << '"/>')
+    def to_xml_string(str = +'')
+      str << (+'<c:axId val="' << @id.to_s << '"/>')
       @scaling.to_xml_string str
-      str << ('<c:delete val="' << @delete.to_s << '"/>')
-      str << ('<c:axPos val="' << @ax_pos.to_s << '"/>')
+      str << (+'<c:delete val="' << @delete.to_s << '"/>')
+      str << (+'<c:axPos val="' << @ax_pos.to_s << '"/>')
       str << '<c:majorGridlines>'
       # TODO: shape properties need to be extracted into a class
       if gridlines == false
@@ -167,21 +167,21 @@ module Axlsx
       # Need to set sourceLinked to 0 if we're setting a format code on this row
       # otherwise it will never take, as it will always prefer the 'General' formatting
       # of the cells themselves
-      str << ('<c:numFmt formatCode="' << @format_code << '" sourceLinked="' << (@format_code.eql?('General') ? '1' : '0') << '"/>')
+      str << (+'<c:numFmt formatCode="' << @format_code << '" sourceLinked="' << (@format_code.eql?('General') ? '1' : '0') << '"/>')
       str << '<c:majorTickMark val="none"/>'
       str << '<c:minorTickMark val="none"/>'
-      str << ('<c:tickLblPos val="' << @tick_lbl_pos.to_s << '"/>')
+      str << (+'<c:tickLblPos val="' << @tick_lbl_pos.to_s << '"/>')
       # TODO: this is also being used for series colors
       # time to extract this into a class spPr - Shape Properties
       if @color
         str << '<c:spPr><a:ln><a:solidFill>'
-        str << ('<a:srgbClr val="' << @color << '"/>')
+        str << (+'<a:srgbClr val="' << @color << '"/>')
         str << '</a:solidFill></a:ln></c:spPr>'
       end
       # some potential value in implementing this in full. Very detailed!
-      str << ('<c:txPr><a:bodyPr rot="' << @label_rotation.to_s << '"/><a:lstStyle/><a:p><a:pPr><a:defRPr/></a:pPr><a:endParaRPr/></a:p></c:txPr>')
-      str << ('<c:crossAx val="' << @cross_axis.id.to_s << '"/>')
-      str << ('<c:crosses val="' << @crosses.to_s << '"/>')
+      str << (+'<c:txPr><a:bodyPr rot="' << @label_rotation.to_s << '"/><a:lstStyle/><a:p><a:pPr><a:defRPr/></a:pPr><a:endParaRPr/></a:p></c:txPr>')
+      str << (+'<c:crossAx val="' << @cross_axis.id.to_s << '"/>')
+      str << (+'<c:crosses val="' << @crosses.to_s << '"/>')
     end
   end
 end
diff --git a/lib/axlsx/drawing/bar_3D_chart.rb b/lib/axlsx/drawing/bar_3D_chart.rb
index 0ba55eb2..37ac7dab 100644
--- a/lib/axlsx/drawing/bar_3D_chart.rb
+++ b/lib/axlsx/drawing/bar_3D_chart.rb
@@ -120,17 +120,17 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
         str << '<c:bar3DChart>'
-        str << ('<c:barDir val="' << bar_dir.to_s << '"/>')
-        str << ('<c:grouping val="' << grouping.to_s << '"/>')
-        str << ('<c:varyColors val="' << vary_colors.to_s << '"/>')
+        str << (+'<c:barDir val="' << bar_dir.to_s << '"/>')
+        str << (+'<c:grouping val="' << grouping.to_s << '"/>')
+        str << (+'<c:varyColors val="' << vary_colors.to_s << '"/>')
         @series.each { |ser| ser.to_xml_string(str) }
         @d_lbls.to_xml_string(str) if @d_lbls
-        str << ('<c:gapWidth val="' << @gap_width.to_s << '"/>') unless @gap_width.nil?
-        str << ('<c:gapDepth val="' << @gap_depth.to_s << '"/>') unless @gap_depth.nil?
-        str << ('<c:shape val="' << @shape.to_s << '"/>') unless @shape.nil?
+        str << (+'<c:gapWidth val="' << @gap_width.to_s << '"/>') unless @gap_width.nil?
+        str << (+'<c:gapDepth val="' << @gap_depth.to_s << '"/>') unless @gap_depth.nil?
+        str << (+'<c:shape val="' << @shape.to_s << '"/>') unless @shape.nil?
         axes.to_xml_string(str, :ids => true)
         str << '</c:bar3DChart>'
         axes.to_xml_string(str)
diff --git a/lib/axlsx/drawing/bar_chart.rb b/lib/axlsx/drawing/bar_chart.rb
index c3240f32..eca54dfc 100644
--- a/lib/axlsx/drawing/bar_chart.rb
+++ b/lib/axlsx/drawing/bar_chart.rb
@@ -110,17 +110,17 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
         str << '<c:barChart>'
-        str << ('<c:barDir val="' << bar_dir.to_s << '"/>')
-        str << ('<c:grouping val="' << grouping.to_s << '"/>')
-        str << ('<c:varyColors val="' << vary_colors.to_s << '"/>')
+        str << (+'<c:barDir val="' << bar_dir.to_s << '"/>')
+        str << (+'<c:grouping val="' << grouping.to_s << '"/>')
+        str << (+'<c:varyColors val="' << vary_colors.to_s << '"/>')
         @series.each { |ser| ser.to_xml_string(str) }
         @d_lbls.to_xml_string(str) if @d_lbls
-        str << ('<c:overlap val="' << @overlap.to_s << '"/>') unless @overlap.nil?
-        str << ('<c:gapWidth val="' << @gap_width.to_s << '"/>') unless @gap_width.nil?
-        str << ('<c:shape val="' << @shape.to_s << '"/>') unless @shape.nil?
+        str << (+'<c:overlap val="' << @overlap.to_s << '"/>') unless @overlap.nil?
+        str << (+'<c:gapWidth val="' << @gap_width.to_s << '"/>') unless @gap_width.nil?
+        str << (+'<c:shape val="' << @shape.to_s << '"/>') unless @shape.nil?
         axes.to_xml_string(str, :ids => true)
         str << '</c:barChart>'
         axes.to_xml_string(str)
diff --git a/lib/axlsx/drawing/bar_series.rb b/lib/axlsx/drawing/bar_series.rb
index 86ca4e88..c744175f 100644
--- a/lib/axlsx/drawing/bar_series.rb
+++ b/lib/axlsx/drawing/bar_series.rb
@@ -58,19 +58,19 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
         colors.each_with_index do |c, index|
           str << '<c:dPt>'
-          str << ('<c:idx val="' << index.to_s << '"/>')
+          str << (+'<c:idx val="' << index.to_s << '"/>')
           str << '<c:spPr><a:solidFill>'
-          str << ('<a:srgbClr val="' << c << '"/>')
+          str << (+'<a:srgbClr val="' << c << '"/>')
           str << '</a:solidFill></c:spPr></c:dPt>'
         end
 
         if series_color
           str << '<c:spPr><a:solidFill>'
-          str << ('<a:srgbClr val="' << series_color << '"/>')
+          str << (+'<a:srgbClr val="' << series_color << '"/>')
           str << '</a:solidFill>'
           str << '</c:spPr>'
         end
@@ -78,7 +78,7 @@ module Axlsx
         @labels.to_xml_string(str) unless @labels.nil?
         @data.to_xml_string(str) unless @data.nil?
         # this is actually only required for shapes other than box
-        str << ('<c:shape val="' << shape.to_s << '"></c:shape>')
+        str << (+'<c:shape val="' << shape.to_s << '"></c:shape>')
       end
     end
 
diff --git a/lib/axlsx/drawing/bubble_chart.rb b/lib/axlsx/drawing/bubble_chart.rb
index 73b74035..b4932042 100644
--- a/lib/axlsx/drawing/bubble_chart.rb
+++ b/lib/axlsx/drawing/bubble_chart.rb
@@ -35,10 +35,10 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
         str << '<c:bubbleChart>'
-        str << ('<c:varyColors val="' << vary_colors.to_s << '"/>')
+        str << (+'<c:varyColors val="' << vary_colors.to_s << '"/>')
         @series.each { |ser| ser.to_xml_string(str) }
         d_lbls.to_xml_string(str) if @d_lbls
         axes.to_xml_string(str, :ids => true)
diff --git a/lib/axlsx/drawing/bubble_series.rb b/lib/axlsx/drawing/bubble_series.rb
index e44a3fd8..25335270 100644
--- a/lib/axlsx/drawing/bubble_series.rb
+++ b/lib/axlsx/drawing/bubble_series.rb
@@ -41,15 +41,15 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
         # needs to override the super color here to push in ln/and something else!
         if color
           str << '<c:spPr><a:solidFill>'
-          str << ('<a:srgbClr val="' << color << '"/>')
+          str << (+'<a:srgbClr val="' << color << '"/>')
           str << '</a:solidFill>'
           str << '<a:ln><a:solidFill>'
-          str << ('<a:srgbClr val="' << color << '"/></a:solidFill></a:ln>')
+          str << (+'<a:srgbClr val="' << color << '"/></a:solidFill></a:ln>')
           str << '</c:spPr>'
         end
         @xData.to_xml_string(str) unless @xData.nil?
diff --git a/lib/axlsx/drawing/cat_axis.rb b/lib/axlsx/drawing/cat_axis.rb
index 23568d6f..a36f62d1 100644
--- a/lib/axlsx/drawing/cat_axis.rb
+++ b/lib/axlsx/drawing/cat_axis.rb
@@ -68,14 +68,14 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<c:catAx>'
       super(str)
-      str << ('<c:auto val="' << @auto.to_s << '"/>')
-      str << ('<c:lblAlgn val="' << @lbl_algn.to_s << '"/>')
-      str << ('<c:lblOffset val="' << @lbl_offset.to_i.to_s << '"/>')
-      str << ('<c:tickLblSkip val="' << @tick_lbl_skip.to_s << '"/>')
-      str << ('<c:tickMarkSkip val="' << @tick_mark_skip.to_s << '"/>')
+      str << (+'<c:auto val="' << @auto.to_s << '"/>')
+      str << (+'<c:lblAlgn val="' << @lbl_algn.to_s << '"/>')
+      str << (+'<c:lblOffset val="' << @lbl_offset.to_i.to_s << '"/>')
+      str << (+'<c:tickLblSkip val="' << @tick_lbl_skip.to_s << '"/>')
+      str << (+'<c:tickMarkSkip val="' << @tick_mark_skip.to_s << '"/>')
       str << '</c:catAx>'
     end
   end
diff --git a/lib/axlsx/drawing/chart.rb b/lib/axlsx/drawing/chart.rb
index 636c1713..300a5086 100644
--- a/lib/axlsx/drawing/chart.rb
+++ b/lib/axlsx/drawing/chart.rb
@@ -204,15 +204,15 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<?xml version="1.0" encoding="UTF-8"?>'
-      str << ('<c:chartSpace xmlns:c="' << XML_NS_C << '" xmlns:a="' << XML_NS_A << '" xmlns:r="' << XML_NS_R << '">')
-      str << ('<c:date1904 val="' << Axlsx::Workbook.date1904.to_s << '"/>')
-      str << ('<c:roundedCorners val="' << rounded_corners.to_s << '"/>')
-      str << ('<c:style val="' << style.to_s << '"/>')
+      str << (+'<c:chartSpace xmlns:c="' << XML_NS_C << '" xmlns:a="' << XML_NS_A << '" xmlns:r="' << XML_NS_R << '">')
+      str << (+'<c:date1904 val="' << Axlsx::Workbook.date1904.to_s << '"/>')
+      str << (+'<c:roundedCorners val="' << rounded_corners.to_s << '"/>')
+      str << (+'<c:style val="' << style.to_s << '"/>')
       str << '<c:chart>'
       @title.to_xml_string(str) unless @title.empty?
-      str << ('<c:autoTitleDeleted val="' << @title.nil?.to_s << '"/>')
+      str << (+'<c:autoTitleDeleted val="' << @title.nil?.to_s << '"/>')
       @view_3D.to_xml_string(str) if @view_3D
       str << '<c:floor><c:thickness val="0"/></c:floor>'
       str << '<c:sideWall><c:thickness val="0"/></c:sideWall>'
@@ -223,13 +223,13 @@ module Axlsx
       str << '</c:plotArea>'
       if @show_legend
         str << '<c:legend>'
-        str << ('<c:legendPos val="' << @legend_position.to_s << '"/>')
+        str << (+'<c:legendPos val="' << @legend_position.to_s << '"/>')
         str << '<c:layout/>'
         str << '<c:overlay val="0"/>'
         str << '</c:legend>'
       end
-      str << ('<c:plotVisOnly val="' << @plot_visible_only.to_s << '"/>')
-      str << ('<c:dispBlanksAs val="' << display_blanks_as.to_s << '"/>')
+      str << (+'<c:plotVisOnly val="' << @plot_visible_only.to_s << '"/>')
+      str << (+'<c:dispBlanksAs val="' << display_blanks_as.to_s << '"/>')
       str << '<c:showDLblsOverMax val="1"/>'
       str << '</c:chart>'
       if bg_color
diff --git a/lib/axlsx/drawing/d_lbls.rb b/lib/axlsx/drawing/d_lbls.rb
index cdccf327..4e2bd40c 100644
--- a/lib/axlsx/drawing/d_lbls.rb
+++ b/lib/axlsx/drawing/d_lbls.rb
@@ -71,7 +71,7 @@ module Axlsx
 
     # serializes the data labels
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       validate_attributes_for_chart_type
       str << '<c:dLbls>'
       instance_vals = Axlsx.instance_values_for(self)
diff --git a/lib/axlsx/drawing/drawing.rb b/lib/axlsx/drawing/drawing.rb
index d36e4349..7be169be 100644
--- a/lib/axlsx/drawing/drawing.rb
+++ b/lib/axlsx/drawing/drawing.rb
@@ -155,9 +155,9 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<?xml version="1.0" encoding="UTF-8" standalone="yes"?>'
-      str << ('<xdr:wsDr xmlns:xdr="' << XML_NS_XDR << '" xmlns:a="' << XML_NS_A << '">')
+      str << (+'<xdr:wsDr xmlns:xdr="' << XML_NS_XDR << '" xmlns:a="' << XML_NS_A << '">')
       anchors.each { |anchor| anchor.to_xml_string(str) }
       str << '</xdr:wsDr>'
     end
diff --git a/lib/axlsx/drawing/graphic_frame.rb b/lib/axlsx/drawing/graphic_frame.rb
index e245b9ab..7d0e818c 100644
--- a/lib/axlsx/drawing/graphic_frame.rb
+++ b/lib/axlsx/drawing/graphic_frame.rb
@@ -31,11 +31,11 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       # macro attribute should be optional!
       str << '<xdr:graphicFrame>'
       str << '<xdr:nvGraphicFramePr>'
-      str << ('<xdr:cNvPr id="' << @anchor.drawing.index.to_s << '" name="' << 'item_' << @anchor.drawing.index.to_s << '"/>')
+      str << (+'<xdr:cNvPr id="' << @anchor.drawing.index.to_s << '" name="' << 'item_' << @anchor.drawing.index.to_s << '"/>')
       str << '<xdr:cNvGraphicFramePr/>'
       str << '</xdr:nvGraphicFramePr>'
       str << '<xdr:xfrm>'
@@ -43,8 +43,8 @@ module Axlsx
       str << '<a:ext cx="0" cy="0"/>'
       str << '</xdr:xfrm>'
       str << '<a:graphic>'
-      str << ('<a:graphicData uri="' << XML_NS_C << '">')
-      str << ('<c:chart xmlns:c="' << XML_NS_C << '" xmlns:r="' << XML_NS_R << '" r:id="' << rId << '"/>')
+      str << (+'<a:graphicData uri="' << XML_NS_C << '">')
+      str << (+'<c:chart xmlns:c="' << XML_NS_C << '" xmlns:r="' << XML_NS_R << '" r:id="' << rId << '"/>')
       str << '</a:graphicData>'
       str << '</a:graphic>'
       str << '</xdr:graphicFrame>'
diff --git a/lib/axlsx/drawing/hyperlink.rb b/lib/axlsx/drawing/hyperlink.rb
index 1fd7fdd4..3fb97069 100644
--- a/lib/axlsx/drawing/hyperlink.rb
+++ b/lib/axlsx/drawing/hyperlink.rb
@@ -92,7 +92,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag 'a:hlinkClick', str, { :'r:id' => relationship.Id, :'xmlns:r' => XML_NS_R }
     end
   end
diff --git a/lib/axlsx/drawing/line_3D_chart.rb b/lib/axlsx/drawing/line_3D_chart.rb
index 51beaa70..6a659550 100644
--- a/lib/axlsx/drawing/line_3D_chart.rb
+++ b/lib/axlsx/drawing/line_3D_chart.rb
@@ -57,9 +57,9 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
-        str << ('<c:gapDepth val="' << @gap_depth.to_s << '"/>') unless @gap_depth.nil?
+        str << (+'<c:gapDepth val="' << @gap_depth.to_s << '"/>') unless @gap_depth.nil?
       end
     end
   end
diff --git a/lib/axlsx/drawing/line_chart.rb b/lib/axlsx/drawing/line_chart.rb
index 87757dbb..ab2012c7 100644
--- a/lib/axlsx/drawing/line_chart.rb
+++ b/lib/axlsx/drawing/line_chart.rb
@@ -74,16 +74,16 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
-        str << ("<c:" << node_name << ">")
-        str << ('<c:grouping val="' << grouping.to_s << '"/>')
-        str << ('<c:varyColors val="' << vary_colors.to_s << '"/>')
+        str << (+"<c:" << node_name << ">")
+        str << (+'<c:grouping val="' << grouping.to_s << '"/>')
+        str << (+'<c:varyColors val="' << vary_colors.to_s << '"/>')
         @series.each { |ser| ser.to_xml_string(str) }
         @d_lbls.to_xml_string(str) if @d_lbls
         yield if block_given?
         axes.to_xml_string(str, :ids => true)
-        str << ("</c:" << node_name << ">")
+        str << (+"</c:" << node_name << ">")
         axes.to_xml_string(str)
       end
     end
diff --git a/lib/axlsx/drawing/line_series.rb b/lib/axlsx/drawing/line_series.rb
index b1e36bbe..42c86873 100644
--- a/lib/axlsx/drawing/line_series.rb
+++ b/lib/axlsx/drawing/line_series.rb
@@ -71,15 +71,15 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
         if color
           str << '<c:spPr><a:solidFill>'
-          str << ('<a:srgbClr val="' << color << '"/>')
+          str << (+'<a:srgbClr val="' << color << '"/>')
           str << '</a:solidFill>'
           str << '<a:ln w="28800">'
           str << '<a:solidFill>'
-          str << ('<a:srgbClr val="' << color << '"/>')
+          str << (+'<a:srgbClr val="' << color << '"/>')
           str << '</a:solidFill>'
           str << '</a:ln>'
           str << '<a:round/>'
@@ -94,7 +94,7 @@ module Axlsx
 
         @labels.to_xml_string(str) unless @labels.nil?
         @data.to_xml_string(str) unless @data.nil?
-        str << ('<c:smooth val="' << ((smooth) ? '1' : '0') << '"/>')
+        str << (+'<c:smooth val="' << ((smooth) ? '1' : '0') << '"/>')
       end
     end
 
diff --git a/lib/axlsx/drawing/marker.rb b/lib/axlsx/drawing/marker.rb
index 0e728fc6..987af605 100644
--- a/lib/axlsx/drawing/marker.rb
+++ b/lib/axlsx/drawing/marker.rb
@@ -56,9 +56,9 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       [:col, :colOff, :row, :rowOff].each do |k|
-        str << ('<xdr:' << k.to_s << '>' << self.send(k).to_s << '</xdr:' << k.to_s << '>')
+        str << (+'<xdr:' << k.to_s << '>' << self.send(k).to_s << '</xdr:' << k.to_s << '>')
       end
     end
 
diff --git a/lib/axlsx/drawing/num_data.rb b/lib/axlsx/drawing/num_data.rb
index 5c4e832f..79108777 100644
--- a/lib/axlsx/drawing/num_data.rb
+++ b/lib/axlsx/drawing/num_data.rb
@@ -36,14 +36,14 @@ module Axlsx
     end
 
     # serialize the object
-    def to_xml_string(str = "")
-      str << ('<c:' << @tag_name.to_s << '>')
-      str << ('<c:formatCode>' << format_code.to_s << '</c:formatCode>')
-      str << ('<c:ptCount val="' << @pt.size.to_s << '"/>')
+    def to_xml_string(str = +'')
+      str << (+'<c:' << @tag_name.to_s << '>')
+      str << (+'<c:formatCode>' << format_code.to_s << '</c:formatCode>')
+      str << (+'<c:ptCount val="' << @pt.size.to_s << '"/>')
       @pt.each_with_index do |num_val, index|
         num_val.to_xml_string index, str
       end
-      str << ('</c:' << @tag_name.to_s << '>')
+      str << (+'</c:' << @tag_name.to_s << '>')
     end
   end
 end
diff --git a/lib/axlsx/drawing/num_data_source.rb b/lib/axlsx/drawing/num_data_source.rb
index 0159a9d6..af5e31ab 100644
--- a/lib/axlsx/drawing/num_data_source.rb
+++ b/lib/axlsx/drawing/num_data_source.rb
@@ -44,17 +44,17 @@ module Axlsx
 
     # serialize the object
     # @param [String] str
-    def to_xml_string(str = "")
-      str << ('<c:' << tag_name.to_s << '>')
+    def to_xml_string(str = +'')
+      str << (+'<c:' << tag_name.to_s << '>')
       if @f
-        str << ('<c:' << @ref_tag_name.to_s << '>')
-        str << ('<c:f>' << @f.to_s << '</c:f>')
+        str << (+'<c:' << @ref_tag_name.to_s << '>')
+        str << (+'<c:f>' << @f.to_s << '</c:f>')
       end
       @data.to_xml_string str
       if @f
-        str << ('</c:' << @ref_tag_name.to_s << '>')
+        str << (+'</c:' << @ref_tag_name.to_s << '>')
       end
-      str << ('</c:' << tag_name.to_s << '>')
+      str << (+'</c:' << tag_name.to_s << '>')
     end
   end
 end
diff --git a/lib/axlsx/drawing/num_val.rb b/lib/axlsx/drawing/num_val.rb
index 3ec9f87b..b614f561 100644
--- a/lib/axlsx/drawing/num_val.rb
+++ b/lib/axlsx/drawing/num_val.rb
@@ -23,10 +23,10 @@ module Axlsx
     end
 
     # serialize the object
-    def to_xml_string(idx, str = "")
+    def to_xml_string(idx, str = +'')
       Axlsx::validate_unsigned_int(idx)
       if !v.to_s.empty?
-        str << ('<c:pt idx="' << idx.to_s << '" formatCode="' << format_code << '"><c:v>' << v.to_s << '</c:v></c:pt>')
+        str << (+'<c:pt idx="' << idx.to_s << '" formatCode="' << format_code << '"><c:v>' << v.to_s << '</c:v></c:pt>')
       end
     end
   end
diff --git a/lib/axlsx/drawing/one_cell_anchor.rb b/lib/axlsx/drawing/one_cell_anchor.rb
index d9999be5..f98d65a1 100644
--- a/lib/axlsx/drawing/one_cell_anchor.rb
+++ b/lib/axlsx/drawing/one_cell_anchor.rb
@@ -74,12 +74,12 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<xdr:oneCellAnchor>'
       str << '<xdr:from>'
       from.to_xml_string(str)
       str << '</xdr:from>'
-      str << ('<xdr:ext cx="' << ext[:cx].to_s << '" cy="' << ext[:cy].to_s << '"/>')
+      str << (+'<xdr:ext cx="' << ext[:cx].to_s << '" cy="' << ext[:cy].to_s << '"/>')
       @object.to_xml_string(str)
       str << '<xdr:clientData/>'
       str << '</xdr:oneCellAnchor>'
diff --git a/lib/axlsx/drawing/pic.rb b/lib/axlsx/drawing/pic.rb
index 1f37cd0f..eec92a9c 100644
--- a/lib/axlsx/drawing/pic.rb
+++ b/lib/axlsx/drawing/pic.rb
@@ -189,10 +189,10 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<xdr:pic>'
       str << '<xdr:nvPicPr>'
-      str << ('<xdr:cNvPr id="2" name="' << name.to_s << '" descr="' << descr.to_s << '">')
+      str << (+'<xdr:cNvPr id="2" name="' << name.to_s << '" descr="' << descr.to_s << '">')
       hyperlink.to_xml_string(str) if hyperlink.is_a?(Hyperlink)
       str << '</xdr:cNvPr><xdr:cNvPicPr>'
       picture_locking.to_xml_string(str)
@@ -213,9 +213,9 @@ module Axlsx
     # Return correct xml relationship string portion
     def relationship_xml_portion
       if remote?
-        ('<a:blip xmlns:r ="' << XML_NS_R << '" r:link="' << relationship.Id << '">')
+        (+'<a:blip xmlns:r ="' << XML_NS_R << '" r:link="' << relationship.Id << '">')
       else
-        ('<a:blip xmlns:r ="' << XML_NS_R << '" r:embed="' << relationship.Id << '">')
+        (+'<a:blip xmlns:r ="' << XML_NS_R << '" r:embed="' << relationship.Id << '">')
       end
     end
 
diff --git a/lib/axlsx/drawing/picture_locking.rb b/lib/axlsx/drawing/picture_locking.rb
index 08ac4527..9511cb15 100644
--- a/lib/axlsx/drawing/picture_locking.rb
+++ b/lib/axlsx/drawing/picture_locking.rb
@@ -34,7 +34,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('a:picLocks', str)
     end
   end
diff --git a/lib/axlsx/drawing/pie_3D_chart.rb b/lib/axlsx/drawing/pie_3D_chart.rb
index be6e3ba7..1c68dbb7 100644
--- a/lib/axlsx/drawing/pie_3D_chart.rb
+++ b/lib/axlsx/drawing/pie_3D_chart.rb
@@ -31,10 +31,10 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
         str << '<c:pie3DChart>'
-        str << ('<c:varyColors val="' << vary_colors.to_s << '"/>')
+        str << (+'<c:varyColors val="' << vary_colors.to_s << '"/>')
         @series.each { |ser| ser.to_xml_string(str) }
         d_lbls.to_xml_string(str) if @d_lbls
         str << '</c:pie3DChart>'
diff --git a/lib/axlsx/drawing/pie_series.rb b/lib/axlsx/drawing/pie_series.rb
index 9cc50aa2..52cd2b66 100644
--- a/lib/axlsx/drawing/pie_series.rb
+++ b/lib/axlsx/drawing/pie_series.rb
@@ -44,14 +44,14 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
         str << '<c:explosion val="' + @explosion.to_s + '"/>' unless @explosion.nil?
         colors.each_with_index do |c, index|
           str << '<c:dPt>'
-          str << ('<c:idx val="' << index.to_s << '"/>')
+          str << (+'<c:idx val="' << index.to_s << '"/>')
           str << '<c:spPr><a:solidFill>'
-          str << ('<a:srgbClr val="' << c << '"/>')
+          str << (+'<a:srgbClr val="' << c << '"/>')
           str << '</a:solidFill></c:spPr></c:dPt>'
         end
         @labels.to_xml_string str unless @labels.nil?
diff --git a/lib/axlsx/drawing/scaling.rb b/lib/axlsx/drawing/scaling.rb
index 4443f880..fc4b9d10 100644
--- a/lib/axlsx/drawing/scaling.rb
+++ b/lib/axlsx/drawing/scaling.rb
@@ -47,12 +47,12 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<c:scaling>'
-      str << ('<c:logBase val="' << @logBase.to_s << '"/>') unless @logBase.nil?
-      str << ('<c:orientation val="' << @orientation.to_s << '"/>') unless @orientation.nil?
-      str << ('<c:min val="' << @min.to_s << '"/>') unless @min.nil?
-      str << ('<c:max val="' << @max.to_s << '"/>') unless @max.nil?
+      str << (+'<c:logBase val="' << @logBase.to_s << '"/>') unless @logBase.nil?
+      str << (+'<c:orientation val="' << @orientation.to_s << '"/>') unless @orientation.nil?
+      str << (+'<c:min val="' << @min.to_s << '"/>') unless @min.nil?
+      str << (+'<c:max val="' << @max.to_s << '"/>') unless @max.nil?
       str << '</c:scaling>'
     end
   end
diff --git a/lib/axlsx/drawing/scatter_chart.rb b/lib/axlsx/drawing/scatter_chart.rb
index 37d528ec..b518c594 100644
--- a/lib/axlsx/drawing/scatter_chart.rb
+++ b/lib/axlsx/drawing/scatter_chart.rb
@@ -49,11 +49,11 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
         str << '<c:scatterChart>'
-        str << ('<c:scatterStyle val="' << scatter_style.to_s << '"/>')
-        str << ('<c:varyColors val="' << vary_colors.to_s << '"/>')
+        str << (+'<c:scatterStyle val="' << scatter_style.to_s << '"/>')
+        str << (+'<c:varyColors val="' << vary_colors.to_s << '"/>')
         @series.each { |ser| ser.to_xml_string(str) }
         d_lbls.to_xml_string(str) if @d_lbls
         axes.to_xml_string(str, :ids => true)
diff --git a/lib/axlsx/drawing/scatter_series.rb b/lib/axlsx/drawing/scatter_series.rb
index 9ea9b8a7..e52306f5 100644
--- a/lib/axlsx/drawing/scatter_series.rb
+++ b/lib/axlsx/drawing/scatter_series.rb
@@ -80,22 +80,22 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       super(str) do
         # needs to override the super color here to push in ln/and something else!
         if color
           str << '<c:spPr><a:solidFill>'
-          str << ('<a:srgbClr val="' << color << '"/>')
+          str << (+'<a:srgbClr val="' << color << '"/>')
           str << '</a:solidFill>'
           str << '<a:ln><a:solidFill>'
-          str << ('<a:srgbClr val="' << color << '"/></a:solidFill></a:ln>')
+          str << (+'<a:srgbClr val="' << color << '"/></a:solidFill></a:ln>')
           str << '</c:spPr>'
           str << '<c:marker>'
           str << '<c:spPr><a:solidFill>'
-          str << ('<a:srgbClr val="' << color << '"/>')
+          str << (+'<a:srgbClr val="' << color << '"/>')
           str << '</a:solidFill>'
           str << '<a:ln><a:solidFill>'
-          str << ('<a:srgbClr val="' << color << '"/></a:solidFill></a:ln>')
+          str << (+'<a:srgbClr val="' << color << '"/></a:solidFill></a:ln>')
           str << '</c:spPr>'
           str << marker_symbol_xml
           str << '</c:marker>'
@@ -110,7 +110,7 @@ module Axlsx
         end
         @xData.to_xml_string(str) unless @xData.nil?
         @yData.to_xml_string(str) unless @yData.nil?
-        str << ('<c:smooth val="' << ((smooth) ? '1' : '0') << '"/>')
+        str << (+'<c:smooth val="' << ((smooth) ? '1' : '0') << '"/>')
       end
       str
     end
diff --git a/lib/axlsx/drawing/ser_axis.rb b/lib/axlsx/drawing/ser_axis.rb
index 4834c355..63628b14 100644
--- a/lib/axlsx/drawing/ser_axis.rb
+++ b/lib/axlsx/drawing/ser_axis.rb
@@ -32,11 +32,11 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<c:serAx>'
       super(str)
-      str << ('<c:tickLblSkip val="' << @tick_lbl_skip.to_s << '"/>') unless @tick_lbl_skip.nil?
-      str << ('<c:tickMarkSkip val="' << @tick_mark_skip.to_s << '"/>') unless @tick_mark_skip.nil?
+      str << (+'<c:tickLblSkip val="' << @tick_lbl_skip.to_s << '"/>') unless @tick_lbl_skip.nil?
+      str << (+'<c:tickMarkSkip val="' << @tick_mark_skip.to_s << '"/>') unless @tick_mark_skip.nil?
       str << '</c:serAx>'
     end
   end
diff --git a/lib/axlsx/drawing/series.rb b/lib/axlsx/drawing/series.rb
index 17c5d24e..a468a6b7 100644
--- a/lib/axlsx/drawing/series.rb
+++ b/lib/axlsx/drawing/series.rb
@@ -57,10 +57,10 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<c:ser>'
-      str << ('<c:idx val="' << index.to_s << '"/>')
-      str << ('<c:order val="' << (order || index).to_s << '"/>')
+      str << (+'<c:idx val="' << index.to_s << '"/>')
+      str << (+'<c:order val="' << (order || index).to_s << '"/>')
       title.to_xml_string(str) unless title.nil?
       yield if block_given?
       str << '</c:ser>'
diff --git a/lib/axlsx/drawing/series_title.rb b/lib/axlsx/drawing/series_title.rb
index 124e30a0..2887495c 100644
--- a/lib/axlsx/drawing/series_title.rb
+++ b/lib/axlsx/drawing/series_title.rb
@@ -6,16 +6,16 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       clean_value = Axlsx::trust_input ? @text.to_s : ::CGI.escapeHTML(Axlsx::sanitize(@text.to_s))
 
       str << '<c:tx>'
       str << '<c:strRef>'
-      str << ('<c:f>' << Axlsx::cell_range([@cell]) << '</c:f>')
+      str << (+'<c:f>' << Axlsx::cell_range([@cell]) << '</c:f>')
       str << '<c:strCache>'
       str << '<c:ptCount val="1"/>'
       str << '<c:pt idx="0">'
-      str << ('<c:v>' << clean_value << '</c:v>')
+      str << (+'<c:v>' << clean_value << '</c:v>')
       str << '</c:pt>'
       str << '</c:strCache>'
       str << '</c:strRef>'
diff --git a/lib/axlsx/drawing/str_data.rb b/lib/axlsx/drawing/str_data.rb
index ab1c25ad..0765671f 100644
--- a/lib/axlsx/drawing/str_data.rb
+++ b/lib/axlsx/drawing/str_data.rb
@@ -27,13 +27,13 @@ module Axlsx
     end
 
     # serialize the object
-    def to_xml_string(str = "")
-      str << ('<c:' << @tag_name.to_s << '>')
-      str << ('<c:ptCount val="' << @pt.size.to_s << '"/>')
+    def to_xml_string(str = +'')
+      str << (+'<c:' << @tag_name.to_s << '>')
+      str << (+'<c:ptCount val="' << @pt.size.to_s << '"/>')
       @pt.each_with_index do |value, index|
         value.to_xml_string index, str
       end
-      str << ('</c:' << @tag_name.to_s << '>')
+      str << (+'</c:' << @tag_name.to_s << '>')
     end
   end
 end
diff --git a/lib/axlsx/drawing/str_val.rb b/lib/axlsx/drawing/str_val.rb
index f6d36d41..821c7037 100644
--- a/lib/axlsx/drawing/str_val.rb
+++ b/lib/axlsx/drawing/str_val.rb
@@ -23,10 +23,10 @@ module Axlsx
     end
 
     # serialize the object
-    def to_xml_string(idx, str = "")
+    def to_xml_string(idx, str = +'')
       Axlsx::validate_unsigned_int(idx)
       if !v.to_s.empty?
-        str << ('<c:pt idx="' << idx.to_s << '"><c:v>' << ::CGI.escapeHTML(v.to_s) << '</c:v></c:pt>')
+        str << (+'<c:pt idx="' << idx.to_s << '"><c:v>' << ::CGI.escapeHTML(v.to_s) << '</c:v></c:pt>')
       end
     end
   end
diff --git a/lib/axlsx/drawing/title.rb b/lib/axlsx/drawing/title.rb
index 9fd90d29..7bd41b17 100644
--- a/lib/axlsx/drawing/title.rb
+++ b/lib/axlsx/drawing/title.rb
@@ -69,18 +69,18 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<c:title>'
       unless empty?
         clean_value = Axlsx::trust_input ? @text.to_s : ::CGI.escapeHTML(Axlsx::sanitize(@text.to_s))
         str << '<c:tx>'
         if @cell.is_a?(Cell)
           str << '<c:strRef>'
-          str << ('<c:f>' << Axlsx::cell_range([@cell]) << '</c:f>')
+          str << (+'<c:f>' << Axlsx::cell_range([@cell]) << '</c:f>')
           str << '<c:strCache>'
           str << '<c:ptCount val="1"/>'
           str << '<c:pt idx="0">'
-          str << ('<c:v>' << clean_value << '</c:v>')
+          str << (+'<c:v>' << clean_value << '</c:v>')
           str << '</c:pt>'
           str << '</c:strCache>'
           str << '</c:strRef>'
@@ -90,8 +90,8 @@ module Axlsx
           str << '<a:lstStyle/>'
           str << '<a:p>'
           str << '<a:r>'
-          str << ('<a:rPr sz="' << @text_size.to_s << '"/>')
-          str << ('<a:t>' << clean_value << '</a:t>')
+          str << (+'<a:rPr sz="' << @text_size.to_s << '"/>')
+          str << (+'<a:t>' << clean_value << '</a:t>')
           str << '</a:r>'
           str << '</a:p>'
           str << '</c:rich>'
diff --git a/lib/axlsx/drawing/two_cell_anchor.rb b/lib/axlsx/drawing/two_cell_anchor.rb
index 7b01ae13..bebfd28c 100644
--- a/lib/axlsx/drawing/two_cell_anchor.rb
+++ b/lib/axlsx/drawing/two_cell_anchor.rb
@@ -81,7 +81,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<xdr:twoCellAnchor>'
       str << '<xdr:from>'
       from.to_xml_string str
diff --git a/lib/axlsx/drawing/val_axis.rb b/lib/axlsx/drawing/val_axis.rb
index 8ebdcc92..1200eb5d 100644
--- a/lib/axlsx/drawing/val_axis.rb
+++ b/lib/axlsx/drawing/val_axis.rb
@@ -26,10 +26,10 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<c:valAx>'
       super(str)
-      str << ('<c:crossBetween val="' << @cross_between.to_s << '"/>')
+      str << (+'<c:crossBetween val="' << @cross_between.to_s << '"/>')
       str << '</c:valAx>'
     end
   end
diff --git a/lib/axlsx/drawing/view_3D.rb b/lib/axlsx/drawing/view_3D.rb
index a414b46a..9b1b5cb2 100644
--- a/lib/axlsx/drawing/view_3D.rb
+++ b/lib/axlsx/drawing/view_3D.rb
@@ -96,7 +96,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<c:view3D>'
       %w(rot_x h_percent rot_y depth_percent r_ang_ax perspective).each do |key|
         str << element_for_attribute(key, 'c')
diff --git a/lib/axlsx/drawing/vml_drawing.rb b/lib/axlsx/drawing/vml_drawing.rb
index 0805421e..1da4e51a 100644
--- a/lib/axlsx/drawing/vml_drawing.rb
+++ b/lib/axlsx/drawing/vml_drawing.rb
@@ -20,7 +20,7 @@ module Axlsx
     # serialize the vml_drawing to xml.
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << <<BAD_PROGRAMMER
 <xml xmlns:v="urn:schemas-microsoft-com:vml"
  xmlns:o="urn:schemas-microsoft-com:office:office"
diff --git a/lib/axlsx/drawing/vml_shape.rb b/lib/axlsx/drawing/vml_shape.rb
index 4db280f3..5d225d2a 100644
--- a/lib/axlsx/drawing/vml_shape.rb
+++ b/lib/axlsx/drawing/vml_shape.rb
@@ -37,7 +37,7 @@ module Axlsx
     # serialize the shape to a string
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << <<SHAME_ON_YOU
 
 <v:shape id="#{@id}" type="#_x0000_t202" fillcolor="#ffffa1 [80]" o:insetmode="auto"
diff --git a/lib/axlsx/rels/relationship.rb b/lib/axlsx/rels/relationship.rb
index 3f296b82..d9393b15 100644
--- a/lib/axlsx/rels/relationship.rb
+++ b/lib/axlsx/rels/relationship.rb
@@ -102,10 +102,10 @@ module Axlsx
     # serialize relationship
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       h = Axlsx.instance_values_for(self).reject { |k, _| k == "source_obj" }
       str << '<Relationship '
-      str << (h.map { |key, value| '' << key.to_s << '="' << Axlsx::coder.encode(value.to_s) << '"' }.join(' '))
+      str << (h.map { |key, value| +'' << key.to_s << '="' << Axlsx::coder.encode(value.to_s) << '"' }.join(' '))
       str << '/>'
     end
 
diff --git a/lib/axlsx/rels/relationships.rb b/lib/axlsx/rels/relationships.rb
index 64126ed7..ec5b16e9 100644
--- a/lib/axlsx/rels/relationships.rb
+++ b/lib/axlsx/rels/relationships.rb
@@ -21,9 +21,9 @@ module Axlsx
     # serialize relationships
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<?xml version="1.0" encoding="UTF-8"?>'
-      str << ('<Relationships xmlns="' << RELS_R << '">')
+      str << (+'<Relationships xmlns="' << RELS_R << '">')
       each { |rel| rel.to_xml_string(str) }
       str << '</Relationships>'
     end
diff --git a/lib/axlsx/stylesheet/border.rb b/lib/axlsx/stylesheet/border.rb
index ed172ba8..163479b1 100644
--- a/lib/axlsx/stylesheet/border.rb
+++ b/lib/axlsx/stylesheet/border.rb
@@ -56,7 +56,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<border '
       serialized_attributes str
       str << '>'
diff --git a/lib/axlsx/stylesheet/border_pr.rb b/lib/axlsx/stylesheet/border_pr.rb
index 8c1d2ede..154b25f4 100644
--- a/lib/axlsx/stylesheet/border_pr.rb
+++ b/lib/axlsx/stylesheet/border_pr.rb
@@ -62,10 +62,10 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
-      str << ('<' << @name.to_s << ' style="' << @style.to_s << '">')
+    def to_xml_string(str = +'')
+      str << (+'<' << @name.to_s << ' style="' << @style.to_s << '">')
       @color.to_xml_string(str) if @color.is_a?(Color)
-      str << ('</' << @name.to_s << '>')
+      str << (+'</' << @name.to_s << '>')
     end
   end
 end
diff --git a/lib/axlsx/stylesheet/cell_alignment.rb b/lib/axlsx/stylesheet/cell_alignment.rb
index c5eb383e..4a0c0686 100644
--- a/lib/axlsx/stylesheet/cell_alignment.rb
+++ b/lib/axlsx/stylesheet/cell_alignment.rb
@@ -119,7 +119,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('alignment', str)
     end
   end
diff --git a/lib/axlsx/stylesheet/cell_protection.rb b/lib/axlsx/stylesheet/cell_protection.rb
index 3c75d509..caa01ced 100644
--- a/lib/axlsx/stylesheet/cell_protection.rb
+++ b/lib/axlsx/stylesheet/cell_protection.rb
@@ -33,7 +33,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('protection', str)
     end
   end
diff --git a/lib/axlsx/stylesheet/cell_style.rb b/lib/axlsx/stylesheet/cell_style.rb
index 3d198afd..5dc9a90d 100644
--- a/lib/axlsx/stylesheet/cell_style.rb
+++ b/lib/axlsx/stylesheet/cell_style.rb
@@ -63,7 +63,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('cellStyle', str)
     end
   end
diff --git a/lib/axlsx/stylesheet/color.rb b/lib/axlsx/stylesheet/color.rb
index a1743cb2..47617da5 100644
--- a/lib/axlsx/stylesheet/color.rb
+++ b/lib/axlsx/stylesheet/color.rb
@@ -72,7 +72,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '', tag_name = 'color')
+    def to_xml_string(str = +'', tag_name = 'color')
       serialized_tag('' + tag_name + '', str)
     end
   end
diff --git a/lib/axlsx/stylesheet/dxf.rb b/lib/axlsx/stylesheet/dxf.rb
index 0bbae6ca..8c32a75f 100644
--- a/lib/axlsx/stylesheet/dxf.rb
+++ b/lib/axlsx/stylesheet/dxf.rb
@@ -64,7 +64,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<dxf>'
       # Dxf elements have no attributes. All of the instance variables
       # are child elements.
diff --git a/lib/axlsx/stylesheet/fill.rb b/lib/axlsx/stylesheet/fill.rb
index e6561e26..313a24a3 100644
--- a/lib/axlsx/stylesheet/fill.rb
+++ b/lib/axlsx/stylesheet/fill.rb
@@ -21,7 +21,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<fill>'
       @fill_type.to_xml_string(str)
       str << '</fill>'
diff --git a/lib/axlsx/stylesheet/font.rb b/lib/axlsx/stylesheet/font.rb
index 920f6f97..463b8fc0 100644
--- a/lib/axlsx/stylesheet/font.rb
+++ b/lib/axlsx/stylesheet/font.rb
@@ -148,10 +148,10 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<font>'
       Axlsx.instance_values_for(self).each do |k, v|
-        v.is_a?(Color) ? v.to_xml_string(str) : (str << ('<' << k.to_s << ' val="' << Axlsx.booleanize(v).to_s << '"/>'))
+        v.is_a?(Color) ? v.to_xml_string(str) : (str << (+'<' << k.to_s << ' val="' << Axlsx.booleanize(v).to_s << '"/>'))
       end
       str << '</font>'
     end
diff --git a/lib/axlsx/stylesheet/gradient_fill.rb b/lib/axlsx/stylesheet/gradient_fill.rb
index 1c58c12a..6cd0a052 100644
--- a/lib/axlsx/stylesheet/gradient_fill.rb
+++ b/lib/axlsx/stylesheet/gradient_fill.rb
@@ -92,7 +92,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<gradientFill '
       serialized_attributes str
       str << '>'
diff --git a/lib/axlsx/stylesheet/gradient_stop.rb b/lib/axlsx/stylesheet/gradient_stop.rb
index fd53a41a..83f39a55 100644
--- a/lib/axlsx/stylesheet/gradient_stop.rb
+++ b/lib/axlsx/stylesheet/gradient_stop.rb
@@ -29,8 +29,8 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
-      str << ('<stop position="' << position.to_s << '">')
+    def to_xml_string(str = +'')
+      str << (+'<stop position="' << position.to_s << '">')
       self.color.to_xml_string(str)
       str << '</stop>'
     end
diff --git a/lib/axlsx/stylesheet/num_fmt.rb b/lib/axlsx/stylesheet/num_fmt.rb
index 1864fcf2..1a7add0e 100644
--- a/lib/axlsx/stylesheet/num_fmt.rb
+++ b/lib/axlsx/stylesheet/num_fmt.rb
@@ -69,12 +69,12 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('numFmt', str)
     end
 
     # Override to avoid removing underscores
-    def serialized_attributes(str = '', additional_attributes = {})
+    def serialized_attributes(str = +'', additional_attributes = {})
       attributes = declared_attributes.merge! additional_attributes
       attributes.each do |key, value|
         str << "#{Axlsx.camel(key, false)}=\"#{Axlsx.booleanize(value)}\" "
diff --git a/lib/axlsx/stylesheet/pattern_fill.rb b/lib/axlsx/stylesheet/pattern_fill.rb
index 5f89955d..cabce9f5 100644
--- a/lib/axlsx/stylesheet/pattern_fill.rb
+++ b/lib/axlsx/stylesheet/pattern_fill.rb
@@ -58,8 +58,8 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
-      str << ('<patternFill patternType="' << patternType.to_s << '">')
+    def to_xml_string(str = +'')
+      str << (+'<patternFill patternType="' << patternType.to_s << '">')
       if fgColor.is_a?(Color)
         fgColor.to_xml_string str, "fgColor"
       end
diff --git a/lib/axlsx/stylesheet/styles.rb b/lib/axlsx/stylesheet/styles.rb
index 3cbe8f75..809630f2 100644
--- a/lib/axlsx/stylesheet/styles.rb
+++ b/lib/axlsx/stylesheet/styles.rb
@@ -485,8 +485,8 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
-      str << ('<styleSheet xmlns="' << XML_NS << '">')
+    def to_xml_string(str = +'')
+      str << (+'<styleSheet xmlns="' << XML_NS << '">')
       instance_vals = Axlsx.instance_values_for(self)
       [:numFmts, :fonts, :fills, :borders, :cellStyleXfs, :cellXfs, :cellStyles, :dxfs, :tableStyles].each do |key|
         instance_vals[key.to_s].to_xml_string(str) unless instance_vals[key.to_s].nil?
diff --git a/lib/axlsx/stylesheet/table_style.rb b/lib/axlsx/stylesheet/table_style.rb
index 24c441ce..4423c518 100644
--- a/lib/axlsx/stylesheet/table_style.rb
+++ b/lib/axlsx/stylesheet/table_style.rb
@@ -42,7 +42,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<tableStyle '
       serialized_attributes str, { :count => self.size }
       str << '>'
diff --git a/lib/axlsx/stylesheet/table_style_element.rb b/lib/axlsx/stylesheet/table_style_element.rb
index c36ab838..2e64d9a3 100644
--- a/lib/axlsx/stylesheet/table_style_element.rb
+++ b/lib/axlsx/stylesheet/table_style_element.rb
@@ -69,7 +69,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('tableStyleElement', str)
     end
   end
diff --git a/lib/axlsx/stylesheet/table_styles.rb b/lib/axlsx/stylesheet/table_styles.rb
index 16cc7320..49f6e601 100644
--- a/lib/axlsx/stylesheet/table_styles.rb
+++ b/lib/axlsx/stylesheet/table_styles.rb
@@ -33,7 +33,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<tableStyles '
       serialized_attributes str, { :count => self.size }
       str << '>'
diff --git a/lib/axlsx/stylesheet/xf.rb b/lib/axlsx/stylesheet/xf.rb
index d42880e0..45d362ca 100644
--- a/lib/axlsx/stylesheet/xf.rb
+++ b/lib/axlsx/stylesheet/xf.rb
@@ -134,7 +134,7 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<xf '
       serialized_attributes str
       str << '>'
diff --git a/lib/axlsx/util/serialized_attributes.rb b/lib/axlsx/util/serialized_attributes.rb
index 04b33fa5..b7247e0d 100644
--- a/lib/axlsx/util/serialized_attributes.rb
+++ b/lib/axlsx/util/serialized_attributes.rb
@@ -49,7 +49,7 @@ module Axlsx
     # serialization to.
     # @param [Hash] additional_attributes An option key value hash for
     # defining values that are not serializable attributes list.
-    def serialized_attributes(str = '', additional_attributes = {})
+    def serialized_attributes(str = +'', additional_attributes = {})
       attributes = declared_attributes.merge! additional_attributes
       attributes.each do |key, value|
         str << "#{Axlsx.camel(key, false)}=\"#{Axlsx.camel(Axlsx.booleanize(value), false)}\" "
@@ -73,7 +73,7 @@ module Axlsx
     # @param [String] str The string instance to which serialized data is appended
     # @param [Array] additional_attributes An array of additional attribute names.
     # @return [String] The serialized output.
-    def serialized_element_attributes(str = '', additional_attributes = [], &block)
+    def serialized_element_attributes(str = +'', additional_attributes = [], &block)
       attrs = self.class.xml_element_attributes + additional_attributes
       values = Axlsx.instance_values_for(self)
       attrs.each do |attribute_name|
diff --git a/lib/axlsx/util/simple_typed_list.rb b/lib/axlsx/util/simple_typed_list.rb
index 1a8bbbec..2bd6b648 100644
--- a/lib/axlsx/util/simple_typed_list.rb
+++ b/lib/axlsx/util/simple_typed_list.rb
@@ -171,12 +171,12 @@ module Axlsx
       }, __FILE__, __LINE__ - 4
     end
 
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       classname = @allowed_types[0].name.split('::').last
       el_name = serialize_as.to_s || (classname[0, 1].downcase + classname[1..-1])
-      str << ('<' << el_name << ' count="' << size.to_s << '">')
+      str << (+'<' << el_name << ' count="' << size.to_s << '">')
       each { |item| item.to_xml_string(str) }
-      str << ('</' << el_name << '>')
+      str << (+'</' << el_name << '>')
     end
   end
 end
diff --git a/lib/axlsx/workbook/defined_name.rb b/lib/axlsx/workbook/defined_name.rb
index bcba47ee..4ddcff2c 100644
--- a/lib/axlsx/workbook/defined_name.rb
+++ b/lib/axlsx/workbook/defined_name.rb
@@ -120,12 +120,12 @@ module Axlsx
     serializable_attributes :short_cut_key, :status_bar, :help, :description, :custom_menu, :comment,
                             :workbook_parameter, :publish_to_server, :xlm, :vb_proceedure, :function, :hidden, :local_sheet_id
 
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       raise ArgumentError, 'you must specify the name for this defined name. Please read the documentation for Axlsx::DefinedName for more details' unless name
 
-      str << ('<definedName ' << 'name="' << name << '" ')
+      str << (+'<definedName ' << 'name="' << name << '" ')
       serialized_attributes str
-      str << ('>' << @formula << '</definedName>')
+      str << (+'>' << @formula << '</definedName>')
     end
   end
 end
diff --git a/lib/axlsx/workbook/defined_names.rb b/lib/axlsx/workbook/defined_names.rb
index 9da776fd..e59acf4b 100644
--- a/lib/axlsx/workbook/defined_names.rb
+++ b/lib/axlsx/workbook/defined_names.rb
@@ -11,7 +11,7 @@ module Axlsx
     # Serialize to xml
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return if empty?
 
       str << '<definedNames>'
diff --git a/lib/axlsx/workbook/shared_strings_table.rb b/lib/axlsx/workbook/shared_strings_table.rb
index f0cbf04d..17007b08 100644
--- a/lib/axlsx/workbook/shared_strings_table.rb
+++ b/lib/axlsx/workbook/shared_strings_table.rb
@@ -36,7 +36,7 @@ module Axlsx
       @index = 0
       @xml_space = xml_space
       @unique_cells = {}
-      @shared_xml_string = ""
+      @shared_xml_string = +''
       shareable_cells = cells.flatten.select { |cell| cell.plain_string? || cell.contains_rich_text? }
       @count = shareable_cells.size
       resolve(shareable_cells)
@@ -45,11 +45,11 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       Axlsx::sanitize(@shared_xml_string)
-      str << ('<?xml version="1.0" encoding="UTF-8"?><sst xmlns="' << XML_NS << '"')
-      str << (' count="' << @count.to_s << '" uniqueCount="' << unique_count.to_s << '"')
-      str << (' xml:space="' << xml_space.to_s << '">' << @shared_xml_string << '</sst>')
+      str << (+'<?xml version="1.0" encoding="UTF-8"?><sst xmlns="' << XML_NS << '"')
+      str << (+' count="' << @count.to_s << '" uniqueCount="' << unique_count.to_s << '"')
+      str << (+' xml:space="' << xml_space.to_s << '">' << @shared_xml_string << '</sst>')
     end
 
     private
diff --git a/lib/axlsx/workbook/workbook.rb b/lib/axlsx/workbook/workbook.rb
index bd70a836..ab16a4ed 100644
--- a/lib/axlsx/workbook/workbook.rb
+++ b/lib/axlsx/workbook/workbook.rb
@@ -405,11 +405,11 @@ module Axlsx
     # Serialize the workbook
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       add_worksheet(name: 'Sheet1') unless worksheets.size > 0
       str << '<?xml version="1.0" encoding="UTF-8"?>'
-      str << ('<workbook xmlns="' << XML_NS << '" xmlns:r="' << XML_NS_R << '">')
-      str << ('<workbookPr date1904="' << @@date1904.to_s << '"/>')
+      str << (+'<workbook xmlns="' << XML_NS << '" xmlns:r="' << XML_NS_R << '">')
+      str << (+'<workbookPr date1904="' << @@date1904.to_s << '"/>')
       views.to_xml_string(str)
       str << '<sheets>'
       if is_reversed
@@ -422,7 +422,7 @@ module Axlsx
       unless pivot_tables.empty?
         str << '<pivotCaches>'
         pivot_tables.each do |pivot_table|
-          str << ('<pivotCache cacheId="' << pivot_table.cache_definition.cache_id.to_s << '" r:id="' << pivot_table.cache_definition.rId << '"/>')
+          str << (+'<pivotCache cacheId="' << pivot_table.cache_definition.cache_id.to_s << '" r:id="' << pivot_table.cache_definition.rId << '"/>')
         end
         str << '</pivotCaches>'
       end
diff --git a/lib/axlsx/workbook/workbook_view.rb b/lib/axlsx/workbook/workbook_view.rb
index e992a65e..398563f9 100644
--- a/lib/axlsx/workbook/workbook_view.rb
+++ b/lib/axlsx/workbook/workbook_view.rb
@@ -68,7 +68,7 @@ module Axlsx
     # Serialize the WorkbookView
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<workbookView '
       serialized_attributes str
       str << '></workbookView>'
diff --git a/lib/axlsx/workbook/workbook_views.rb b/lib/axlsx/workbook/workbook_views.rb
index 569533f8..45d146f1 100644
--- a/lib/axlsx/workbook/workbook_views.rb
+++ b/lib/axlsx/workbook/workbook_views.rb
@@ -11,7 +11,7 @@ module Axlsx
     # Serialize to xml
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return if empty?
 
       str << "<bookViews>"
diff --git a/lib/axlsx/workbook/worksheet/auto_filter/auto_filter.rb b/lib/axlsx/workbook/worksheet/auto_filter/auto_filter.rb
index 156d4ae8..5a74d79a 100644
--- a/lib/axlsx/workbook/worksheet/auto_filter/auto_filter.rb
+++ b/lib/axlsx/workbook/worksheet/auto_filter/auto_filter.rb
@@ -69,7 +69,7 @@ module Axlsx
 
     # serialize the object
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return unless range
 
       str << "<autoFilter ref='#{range}'>"
diff --git a/lib/axlsx/workbook/worksheet/auto_filter/filter_column.rb b/lib/axlsx/workbook/worksheet/auto_filter/filter_column.rb
index 92884e97..f014eb18 100644
--- a/lib/axlsx/workbook/worksheet/auto_filter/filter_column.rb
+++ b/lib/axlsx/workbook/worksheet/auto_filter/filter_column.rb
@@ -87,7 +87,7 @@ module Axlsx
     end
 
     # Serialize the object to xml
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << "<filterColumn #{serialized_attributes}>"
       @filter.to_xml_string(str)
       str << "</filterColumn>"
diff --git a/lib/axlsx/workbook/worksheet/auto_filter/filters.rb b/lib/axlsx/workbook/worksheet/auto_filter/filters.rb
index 77cf7b17..0fc5c968 100644
--- a/lib/axlsx/workbook/worksheet/auto_filter/filters.rb
+++ b/lib/axlsx/workbook/worksheet/auto_filter/filters.rb
@@ -76,7 +76,7 @@ module Axlsx
     end
 
     # Serialize the object to xml
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << "<filters #{serialized_attributes}>"
       filter_items.each { |filter| filter.to_xml_string(str) }
       date_group_items.each { |date_group_item| date_group_item.to_xml_string(str) }
@@ -120,7 +120,7 @@ module Axlsx
 
       # Serializes the filter value object
       # @param [String] str The string to concact the serialization information to.
-      def to_xml_string(str = '')
+      def to_xml_string(str = +'')
         str << "<filter val='#{@val}' />"
       end
     end
@@ -237,7 +237,7 @@ module Axlsx
 
       # Serialize the object to xml
       # @param [String] str The string object this serialization will be concatenated to.
-      def to_xml_string(str = '')
+      def to_xml_string(str = +'')
         serialized_tag('dateGroupItem', str)
       end
     end
diff --git a/lib/axlsx/workbook/worksheet/break.rb b/lib/axlsx/workbook/worksheet/break.rb
index c40503be..ca99a661 100644
--- a/lib/axlsx/workbook/worksheet/break.rb
+++ b/lib/axlsx/workbook/worksheet/break.rb
@@ -27,7 +27,7 @@ module Axlsx
     serializable_attributes :id, :min, :max, :man, :pt
 
     # serializes the break to xml
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('brk', str)
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/cell.rb b/lib/axlsx/workbook/worksheet/cell.rb
index 930356e5..929a7c28 100644
--- a/lib/axlsx/workbook/worksheet/cell.rb
+++ b/lib/axlsx/workbook/worksheet/cell.rb
@@ -388,7 +388,7 @@ module Axlsx
     # @param [Integer] c_index The cell index in the row.
     # @param [String] str The string index the cell content will be appended to. Defaults to empty string.
     # @return [String] xml text for the cell
-    def to_xml_string(r_index, c_index, str = '')
+    def to_xml_string(r_index, c_index, str = +'')
       CellSerializer.to_xml_string r_index, c_index, self, str
     end
 
diff --git a/lib/axlsx/workbook/worksheet/cell_serializer.rb b/lib/axlsx/workbook/worksheet/cell_serializer.rb
index 3c8f87dc..b4c16b57 100644
--- a/lib/axlsx/workbook/worksheet/cell_serializer.rb
+++ b/lib/axlsx/workbook/worksheet/cell_serializer.rb
@@ -9,8 +9,8 @@ module Axlsx
       # @param [Integer] column_index The index of the cell's column
       # @param [String] str The string to apend serialization to.
       # @return [String]
-      def to_xml_string(row_index, column_index, cell, str = '')
-        str << ('<c r="' << Axlsx::cell_r(column_index, row_index) << '" s="' << cell.style.to_s << '" ')
+      def to_xml_string(row_index, column_index, cell, str = +'')
+        str << (+'<c r="' << Axlsx::cell_r(column_index, row_index) << '" s="' << cell.style.to_s << '" ')
         return str << '/>' if cell.value.nil?
 
         method = cell.type
@@ -21,7 +21,7 @@ module Axlsx
       # builds an xml text run based on this cells attributes.
       # @param [String] str The string instance this run will be concated to.
       # @return [String]
-      def run_xml_string(cell, str = '')
+      def run_xml_string(cell, str = +'')
         if cell.is_text_run?
           valid = RichTextRun::INLINE_STYLES - [:value, :type]
           data = Hash[Axlsx.instance_values_for(cell).map { |k, v| [k.to_sym, v] }]
@@ -30,7 +30,7 @@ module Axlsx
         elsif cell.contains_rich_text?
           cell.value.to_xml_string(str)
         else
-          str << ('<t>' << cell.clean_value << '</t>')
+          str << (+'<t>' << cell.clean_value << '</t>')
         end
         str
       end
@@ -39,7 +39,7 @@ module Axlsx
       # @param [Cell] cell The cell that is being serialized
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
-      def iso_8601(cell, str = '')
+      def iso_8601(cell, str = +'')
         value_serialization 'd', cell.value, str
       end
 
@@ -47,7 +47,7 @@ module Axlsx
       # @param [Cell] cell The cell that is being serialized
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
-      def date(cell, str = '')
+      def date(cell, str = +'')
         value_serialization false, DateTimeConverter::date_to_serial(cell.value).to_s, str
       end
 
@@ -55,7 +55,7 @@ module Axlsx
       # @param [Cell] cell The cell that is being serialized
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
-      def time(cell, str = '')
+      def time(cell, str = +'')
         value_serialization false, DateTimeConverter::time_to_serial(cell.value).to_s, str
       end
 
@@ -63,7 +63,7 @@ module Axlsx
       # @param [Cell] cell The cell that is being serialized
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
-      def boolean(cell, str = '')
+      def boolean(cell, str = +'')
         value_serialization 'b', cell.value.to_s, str
       end
 
@@ -71,7 +71,7 @@ module Axlsx
       # @param [Cell] cell The cell that is being serialized
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
-      def float(cell, str = '')
+      def float(cell, str = +'')
         numeric cell, str
       end
 
@@ -79,7 +79,7 @@ module Axlsx
       # @param [Cell] cell The cell that is being serialized
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
-      def integer(cell, str = '')
+      def integer(cell, str = +'')
         numeric cell, str
       end
 
@@ -87,25 +87,25 @@ module Axlsx
       # @param [Cell] cell The cell that is being serialized
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
-      def formula_serialization(cell, str = '')
-        str << ('t="str"><f>' << cell.clean_value.to_s.sub('=', '') << '</f>')
-        str << ('<v>' << cell.formula_value.to_s << '</v>') unless cell.formula_value.nil?
+      def formula_serialization(cell, str = +'')
+        str << (+'t="str"><f>' << cell.clean_value.to_s.sub('=', '') << '</f>')
+        str << (+'<v>' << cell.formula_value.to_s << '</v>') unless cell.formula_value.nil?
       end
 
       # Serializes cells that are type array formula
       # @param [Cell] cell The cell that is being serialized
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
-      def array_formula_serialization(cell, str = '')
-        str << ('t="str">' << '<f t="array" ref="' << cell.r << '">' << cell.clean_value.to_s.sub('{=', '').sub(/}$/, '') << '</f>')
-        str << ('<v>' << cell.formula_value.to_s << '</v>') unless cell.formula_value.nil?
+      def array_formula_serialization(cell, str = +'')
+        str << (+'t="str">' << '<f t="array" ref="' << cell.r << '">' << cell.clean_value.to_s.sub('{=', '').sub(/}$/, '') << '</f>')
+        str << (+'<v>' << cell.formula_value.to_s << '</v>') unless cell.formula_value.nil?
       end
 
       # Serializes cells that are type inline_string
       # @param [Cell] cell The cell that is being serialized
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
-      def inline_string_serialization(cell, str = '')
+      def inline_string_serialization(cell, str = +'')
         str << 't="inlineStr"><is>'
         run_xml_string cell, str
         str << '</is>'
@@ -115,7 +115,7 @@ module Axlsx
       # @param [Cell] cell The cell that is being serialized
       # @param [String] str The string the serialized content will be appended to.
       # @return [String]
-      def string(cell, str = '')
+      def string(cell, str = +'')
         if cell.is_array_formula?
           array_formula_serialization cell, str
         elsif cell.is_formula?
@@ -153,13 +153,13 @@ module Axlsx
 
       private
 
-      def numeric(cell, str = '')
+      def numeric(cell, str = +'')
         value_serialization 'n', cell.value, str
       end
 
-      def value_serialization(serialization_type, serialization_value, str = '')
-        str << ('t="' << serialization_type.to_s << '"') if serialization_type
-        str << ('><v>' << serialization_value.to_s << '</v>')
+      def value_serialization(serialization_type, serialization_value, str = +'')
+        str << (+'t="' << serialization_type.to_s << '"') if serialization_type
+        str << (+'><v>' << serialization_value.to_s << '</v>')
       end
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/cfvo.rb b/lib/axlsx/workbook/worksheet/cfvo.rb
index 7feb933d..9aeb55ed 100644
--- a/lib/axlsx/workbook/worksheet/cfvo.rb
+++ b/lib/axlsx/workbook/worksheet/cfvo.rb
@@ -55,7 +55,7 @@ module Axlsx
     # serialize the Csvo object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('cfvo', str)
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/cfvos.rb b/lib/axlsx/workbook/worksheet/cfvos.rb
index 5115577a..3c41953a 100644
--- a/lib/axlsx/workbook/worksheet/cfvos.rb
+++ b/lib/axlsx/workbook/worksheet/cfvos.rb
@@ -11,7 +11,7 @@ module Axlsx
     # Serialize the Cfvo object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       each { |cfvo| cfvo.to_xml_string(str) }
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/col.rb b/lib/axlsx/workbook/worksheet/col.rb
index 10d421e8..539a6007 100644
--- a/lib/axlsx/workbook/worksheet/col.rb
+++ b/lib/axlsx/workbook/worksheet/col.rb
@@ -137,7 +137,7 @@ module Axlsx
     # Serialize this columns data to an xml string
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('col', str)
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/col_breaks.rb b/lib/axlsx/workbook/worksheet/col_breaks.rb
index 4a7b7f94..2e176f0d 100644
--- a/lib/axlsx/workbook/worksheet/col_breaks.rb
+++ b/lib/axlsx/workbook/worksheet/col_breaks.rb
@@ -25,10 +25,10 @@ module Axlsx
     # <colBreaks count="1" manualBreakCount="1">
     # <brk id="3" max="1048575" man="1"/>
     # </colBreaks>
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return if empty?
 
-      str << ('<colBreaks count="' << size.to_s << '" manualBreakCount="' << size.to_s << '">')
+      str << (+'<colBreaks count="' << size.to_s << '" manualBreakCount="' << size.to_s << '">')
       each { |brk| brk.to_xml_string(str) }
       str << '</colBreaks>'
     end
diff --git a/lib/axlsx/workbook/worksheet/color_scale.rb b/lib/axlsx/workbook/worksheet/color_scale.rb
index 222e2f4d..fb84668b 100644
--- a/lib/axlsx/workbook/worksheet/color_scale.rb
+++ b/lib/axlsx/workbook/worksheet/color_scale.rb
@@ -84,7 +84,7 @@ module Axlsx
     # Serialize this color_scale object data to an xml string
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<colorScale>'
       value_objects.to_xml_string(str)
       colors.each { |color| color.to_xml_string(str) }
diff --git a/lib/axlsx/workbook/worksheet/cols.rb b/lib/axlsx/workbook/worksheet/cols.rb
index 76e0fbf7..7dd3f175 100644
--- a/lib/axlsx/workbook/worksheet/cols.rb
+++ b/lib/axlsx/workbook/worksheet/cols.rb
@@ -14,7 +14,7 @@ module Axlsx
     # Serialize the Cols object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return if empty?
 
       str << '<cols>'
diff --git a/lib/axlsx/workbook/worksheet/comment.rb b/lib/axlsx/workbook/worksheet/comment.rb
index b0fd3e3b..1da779aa 100644
--- a/lib/axlsx/workbook/worksheet/comment.rb
+++ b/lib/axlsx/workbook/worksheet/comment.rb
@@ -61,17 +61,17 @@ module Axlsx
     # serialize the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = "")
+    def to_xml_string(str = +'')
       author = @comments.authors[author_index]
-      str << ('<comment ref="' << ref << '" authorId="' << author_index.to_s << '">')
+      str << (+'<comment ref="' << ref << '" authorId="' << author_index.to_s << '">')
       str << '<text>'
       unless author.to_s == ""
         str << '<r><rPr><b/><color indexed="81"/></rPr>'
-        str << ("<t>" << ::CGI.escapeHTML(author.to_s) << ":\n</t></r>")
+        str << (+"<t>" << ::CGI.escapeHTML(author.to_s) << ":\n</t></r>")
       end
       str << '<r>'
       str << '<rPr><color indexed="81"/></rPr>'
-      str << ('<t>' << ::CGI.escapeHTML(text) << '</t></r></text>')
+      str << (+'<t>' << ::CGI.escapeHTML(text) << '</t></r></text>')
       str << '</comment>'
     end
 
diff --git a/lib/axlsx/workbook/worksheet/comments.rb b/lib/axlsx/workbook/worksheet/comments.rb
index 020b29f5..b121ccd7 100644
--- a/lib/axlsx/workbook/worksheet/comments.rb
+++ b/lib/axlsx/workbook/worksheet/comments.rb
@@ -64,11 +64,11 @@ module Axlsx
     # serialize the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = "")
+    def to_xml_string(str = +'')
       str << '<?xml version="1.0" encoding="UTF-8"?>'
-      str << ('<comments xmlns="' << XML_NS << '"><authors>')
+      str << (+'<comments xmlns="' << XML_NS << '"><authors>')
       authors.each do |author|
-        str << ('<author>' << author.to_s << '</author>')
+        str << (+'<author>' << author.to_s << '</author>')
       end
       str << '</authors><commentList>'
       each do |comment|
diff --git a/lib/axlsx/workbook/worksheet/conditional_formatting.rb b/lib/axlsx/workbook/worksheet/conditional_formatting.rb
index d6293ca8..bd5f5bc6 100644
--- a/lib/axlsx/workbook/worksheet/conditional_formatting.rb
+++ b/lib/axlsx/workbook/worksheet/conditional_formatting.rb
@@ -74,8 +74,8 @@ module Axlsx
     #    </conditionalFormatting>
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
-      str << ('<conditionalFormatting sqref="' << sqref << '">')
+    def to_xml_string(str = +'')
+      str << (+'<conditionalFormatting sqref="' << sqref << '">')
       str << rules.collect { |rule| rule.to_xml_string }.join(' ')
       str << '</conditionalFormatting>'
     end
diff --git a/lib/axlsx/workbook/worksheet/conditional_formatting_rule.rb b/lib/axlsx/workbook/worksheet/conditional_formatting_rule.rb
index d3693d7e..1845f327 100644
--- a/lib/axlsx/workbook/worksheet/conditional_formatting_rule.rb
+++ b/lib/axlsx/workbook/worksheet/conditional_formatting_rule.rb
@@ -204,11 +204,11 @@ module Axlsx
     # Serializes the conditional formatting rule
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<cfRule '
       serialized_attributes str
       str << '>'
-      str << ('<formula>' << [*self.formula].join('</formula><formula>') << '</formula>') if @formula
+      str << (+'<formula>' << [*self.formula].join('</formula><formula>') << '</formula>') if @formula
       @color_scale.to_xml_string(str) if @color_scale && @type == :colorScale
       @data_bar.to_xml_string(str) if @data_bar && @type == :dataBar
       @icon_set.to_xml_string(str) if @icon_set && @type == :iconSet
diff --git a/lib/axlsx/workbook/worksheet/conditional_formattings.rb b/lib/axlsx/workbook/worksheet/conditional_formattings.rb
index ba8c02a3..16ee1e99 100644
--- a/lib/axlsx/workbook/worksheet/conditional_formattings.rb
+++ b/lib/axlsx/workbook/worksheet/conditional_formattings.rb
@@ -16,7 +16,7 @@ module Axlsx
     attr_reader :worksheet
 
     # serialize the conditional formattings
-    def to_xml_string(str = "")
+    def to_xml_string(str = +'')
       return if empty?
 
       each { |item| item.to_xml_string(str) }
diff --git a/lib/axlsx/workbook/worksheet/data_bar.rb b/lib/axlsx/workbook/worksheet/data_bar.rb
index 0e505637..108a5c16 100644
--- a/lib/axlsx/workbook/worksheet/data_bar.rb
+++ b/lib/axlsx/workbook/worksheet/data_bar.rb
@@ -107,7 +107,7 @@ module Axlsx
     # Serialize this object to an xml string
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = "")
+    def to_xml_string(str = +'')
       serialized_tag('dataBar', str) do
         value_objects.to_xml_string(str)
         self.color.to_xml_string(str)
diff --git a/lib/axlsx/workbook/worksheet/data_validation.rb b/lib/axlsx/workbook/worksheet/data_validation.rb
index 82868071..1b9cb46a 100644
--- a/lib/axlsx/workbook/worksheet/data_validation.rb
+++ b/lib/axlsx/workbook/worksheet/data_validation.rb
@@ -233,16 +233,16 @@ module Axlsx
     # Serializes the data validation
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       valid_attributes = get_valid_attributes
 
       str << '<dataValidation '
       str << Axlsx.instance_values_for(self).map do |key, value|
-        '' << key << '="' << Axlsx.booleanize(value).to_s << '"' if (valid_attributes.include?(key.to_sym) && !CHILD_ELEMENTS.include?(key.to_sym))
+        +'' << key << '="' << Axlsx.booleanize(value).to_s << '"' if (valid_attributes.include?(key.to_sym) && !CHILD_ELEMENTS.include?(key.to_sym))
       end.join(' ')
       str << '>'
-      str << ('<formula1>' << self.formula1 << '</formula1>') if @formula1 and valid_attributes.include?(:formula1)
-      str << ('<formula2>' << self.formula2 << '</formula2>') if @formula2 and valid_attributes.include?(:formula2)
+      str << (+'<formula1>' << self.formula1 << '</formula1>') if @formula1 and valid_attributes.include?(:formula1)
+      str << (+'<formula2>' << self.formula2 << '</formula2>') if @formula2 and valid_attributes.include?(:formula2)
       str << '</dataValidation>'
     end
 
diff --git a/lib/axlsx/workbook/worksheet/data_validations.rb b/lib/axlsx/workbook/worksheet/data_validations.rb
index dd5f11b9..a8c2a5b3 100644
--- a/lib/axlsx/workbook/worksheet/data_validations.rb
+++ b/lib/axlsx/workbook/worksheet/data_validations.rb
@@ -16,7 +16,7 @@ module Axlsx
     attr_reader :worksheet
 
     # serialize the conditional formattings
-    def to_xml_string(str = "")
+    def to_xml_string(str = +'')
       return if empty?
 
       str << "<dataValidations count='#{size}'>"
diff --git a/lib/axlsx/workbook/worksheet/dimension.rb b/lib/axlsx/workbook/worksheet/dimension.rb
index 356c0396..42e02e72 100644
--- a/lib/axlsx/workbook/worksheet/dimension.rb
+++ b/lib/axlsx/workbook/worksheet/dimension.rb
@@ -36,7 +36,7 @@ module Axlsx
 
     # serialize the object
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return if worksheet.rows.empty?
 
       str << "<dimension ref=\"%s\"></dimension>" % sqref
diff --git a/lib/axlsx/workbook/worksheet/header_footer.rb b/lib/axlsx/workbook/worksheet/header_footer.rb
index b80f4eeb..c60d6678 100644
--- a/lib/axlsx/workbook/worksheet/header_footer.rb
+++ b/lib/axlsx/workbook/worksheet/header_footer.rb
@@ -42,7 +42,7 @@ module Axlsx
     # Serializes the header/footer object.
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('headerFooter', str) do
         serialized_element_attributes(str) do |value|
           value = ::CGI.escapeHTML(value)
diff --git a/lib/axlsx/workbook/worksheet/icon_set.rb b/lib/axlsx/workbook/worksheet/icon_set.rb
index 0179cc5f..c4b5f2ff 100644
--- a/lib/axlsx/workbook/worksheet/icon_set.rb
+++ b/lib/axlsx/workbook/worksheet/icon_set.rb
@@ -63,7 +63,7 @@ module Axlsx
     # Serialize this object to an xml string
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = "")
+    def to_xml_string(str = +'')
       serialized_tag('iconSet', str) do
         @value_objects.each { |cfvo| cfvo.to_xml_string(str) }
       end
diff --git a/lib/axlsx/workbook/worksheet/merged_cells.rb b/lib/axlsx/workbook/worksheet/merged_cells.rb
index 5ecb1a4e..c0ca1ebe 100644
--- a/lib/axlsx/workbook/worksheet/merged_cells.rb
+++ b/lib/axlsx/workbook/worksheet/merged_cells.rb
@@ -28,7 +28,7 @@ module Axlsx
     # serialize the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return if empty?
 
       str << "<mergeCells count='#{size}'>"
diff --git a/lib/axlsx/workbook/worksheet/outline_pr.rb b/lib/axlsx/workbook/worksheet/outline_pr.rb
index fc301a76..4cfeea2d 100644
--- a/lib/axlsx/workbook/worksheet/outline_pr.rb
+++ b/lib/axlsx/workbook/worksheet/outline_pr.rb
@@ -27,7 +27,7 @@ module Axlsx
     # Serialize the object
     # @param [String] str serialized output will be appended to this object if provided.
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << "<outlinePr #{serialized_attributes} />"
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/page_margins.rb b/lib/axlsx/workbook/worksheet/page_margins.rb
index 1e4a34e0..4d133c5a 100644
--- a/lib/axlsx/workbook/worksheet/page_margins.rb
+++ b/lib/axlsx/workbook/worksheet/page_margins.rb
@@ -92,7 +92,7 @@ module Axlsx
     # @return [String]
     # @note For compatibility, this is a noop unless custom margins have been specified.
     # @see #custom_margins_specified?
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('pageMargins', str)
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/page_set_up_pr.rb b/lib/axlsx/workbook/worksheet/page_set_up_pr.rb
index d1c62c9e..9a2b778b 100644
--- a/lib/axlsx/workbook/worksheet/page_set_up_pr.rb
+++ b/lib/axlsx/workbook/worksheet/page_set_up_pr.rb
@@ -37,8 +37,8 @@ module Axlsx
     end
 
     # serialize to xml
-    def to_xml_string(str = '')
-      str << ('<pageSetUpPr ' << serialized_attributes << '/>')
+    def to_xml_string(str = +'')
+      str << (+'<pageSetUpPr ' << serialized_attributes << '/>')
     end
   end
 end
diff --git a/lib/axlsx/workbook/worksheet/page_setup.rb b/lib/axlsx/workbook/worksheet/page_setup.rb
index fa49142d..0bd0cdb8 100644
--- a/lib/axlsx/workbook/worksheet/page_setup.rb
+++ b/lib/axlsx/workbook/worksheet/page_setup.rb
@@ -232,7 +232,7 @@ module Axlsx
     # Serializes the page settings element.
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('pageSetup', str)
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/pane.rb b/lib/axlsx/workbook/worksheet/pane.rb
index f88b732c..a018dc74 100644
--- a/lib/axlsx/workbook/worksheet/pane.rb
+++ b/lib/axlsx/workbook/worksheet/pane.rb
@@ -122,7 +122,7 @@ module Axlsx
     # Serializes the data validation
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       finalize
       serialized_tag 'pane', str
     end
diff --git a/lib/axlsx/workbook/worksheet/pivot_table.rb b/lib/axlsx/workbook/worksheet/pivot_table.rb
index 8c6cb83e..e97b8119 100644
--- a/lib/axlsx/workbook/worksheet/pivot_table.rb
+++ b/lib/axlsx/workbook/worksheet/pivot_table.rb
@@ -186,13 +186,13 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<?xml version="1.0" encoding="UTF-8"?>'
 
-      str << ('<pivotTableDefinition xmlns="' << XML_NS << '" name="' << name << '" cacheId="' << cache_definition.cache_id.to_s << '"' << (data.size <= 1 ? ' dataOnRows="1"' : '') << ' applyNumberFormats="0" applyBorderFormats="0" applyFontFormats="0" applyPatternFormats="0" applyAlignmentFormats="0" applyWidthHeightFormats="1" dataCaption="Data" showMultipleLabel="0" showMemberPropertyTips="0" useAutoFormatting="1" indent="0" compact="0" compactData="0" gridDropZones="1" multipleFieldFilters="0">')
+      str << (+'<pivotTableDefinition xmlns="' << XML_NS << '" name="' << name << '" cacheId="' << cache_definition.cache_id.to_s << '"' << (data.size <= 1 ? ' dataOnRows="1"' : '') << ' applyNumberFormats="0" applyBorderFormats="0" applyFontFormats="0" applyPatternFormats="0" applyAlignmentFormats="0" applyWidthHeightFormats="1" dataCaption="Data" showMultipleLabel="0" showMemberPropertyTips="0" useAutoFormatting="1" indent="0" compact="0" compactData="0" gridDropZones="1" multipleFieldFilters="0">')
 
-      str << ('<location firstDataCol="1" firstDataRow="1" firstHeaderRow="1" ref="' << ref << '"/>')
-      str << ('<pivotFields count="' << header_cells_count.to_s << '">')
+      str << (+'<location firstDataCol="1" firstDataRow="1" firstHeaderRow="1" ref="' << ref << '"/>')
+      str << (+'<pivotFields count="' << header_cells_count.to_s << '">')
 
       header_cell_values.each do |cell_value|
         subtotal = !no_subtotals_on_headers.include?(cell_value)
@@ -205,12 +205,12 @@ module Axlsx
         str << '<rowFields count="1"><field x="-2"/></rowFields>'
         str << '<rowItems count="2"><i><x/></i> <i i="1"><x v="1"/></i></rowItems>'
       else
-        str << ('<rowFields count="' << rows.size.to_s << '">')
+        str << (+'<rowFields count="' << rows.size.to_s << '">')
         rows.each do |row_value|
-          str << ('<field x="' << header_index_of(row_value).to_s << '"/>')
+          str << (+'<field x="' << header_index_of(row_value).to_s << '"/>')
         end
         str << '</rowFields>'
-        str << ('<rowItems count="' << rows.size.to_s << '">')
+        str << (+'<rowItems count="' << rows.size.to_s << '">')
         rows.size.times do |i|
           str << '<i/>'
         end
@@ -229,16 +229,16 @@ module Axlsx
           str << '<colItems count="1"><i/></colItems>'
         end
       else
-        str << ('<colFields count="' << columns.size.to_s << '">')
+        str << (+'<colFields count="' << columns.size.to_s << '">')
         columns.each do |column_value|
-          str << ('<field x="' << header_index_of(column_value).to_s << '"/>')
+          str << (+'<field x="' << header_index_of(column_value).to_s << '"/>')
         end
         str << '</colFields>'
       end
       unless pages.empty?
-        str << ('<pageFields count="' << pages.size.to_s << '">')
+        str << (+'<pageFields count="' << pages.size.to_s << '">')
         pages.each do |page_value|
-          str << ('<pageField fld="' << header_index_of(page_value).to_s << '"/>')
+          str << (+'<pageField fld="' << header_index_of(page_value).to_s << '"/>')
         end
         str << '</pageFields>'
       end
diff --git a/lib/axlsx/workbook/worksheet/pivot_table_cache_definition.rb b/lib/axlsx/workbook/worksheet/pivot_table_cache_definition.rb
index 136e756e..d42c1fda 100644
--- a/lib/axlsx/workbook/worksheet/pivot_table_cache_definition.rb
+++ b/lib/axlsx/workbook/worksheet/pivot_table_cache_definition.rb
@@ -45,15 +45,15 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<?xml version="1.0" encoding="UTF-8"?>'
-      str << ('<pivotCacheDefinition xmlns="' << XML_NS << '" xmlns:r="' << XML_NS_R << '" invalid="1" refreshOnLoad="1" recordCount="0">')
+      str << (+'<pivotCacheDefinition xmlns="' << XML_NS << '" xmlns:r="' << XML_NS_R << '" invalid="1" refreshOnLoad="1" recordCount="0">')
       str << '<cacheSource type="worksheet">'
-      str << ('<worksheetSource ref="' << pivot_table.range << '" sheet="' << pivot_table.data_sheet.name << '"/>')
+      str << (+'<worksheetSource ref="' << pivot_table.range << '" sheet="' << pivot_table.data_sheet.name << '"/>')
       str << '</cacheSource>'
-      str << ('<cacheFields count="' << pivot_table.header_cells_count.to_s << '">')
+      str << (+'<cacheFields count="' << pivot_table.header_cells_count.to_s << '">')
       pivot_table.header_cells.each do |cell|
-        str << ('<cacheField name="' << cell.clean_value << '" numFmtId="0">')
+        str << (+'<cacheField name="' << cell.clean_value << '" numFmtId="0">')
         str <<     '<sharedItems count="0">'
         str <<     '</sharedItems>'
         str << '</cacheField>'
diff --git a/lib/axlsx/workbook/worksheet/print_options.rb b/lib/axlsx/workbook/worksheet/print_options.rb
index e2c73bfa..6e1b2bfd 100644
--- a/lib/axlsx/workbook/worksheet/print_options.rb
+++ b/lib/axlsx/workbook/worksheet/print_options.rb
@@ -33,7 +33,7 @@ module Axlsx
     # @note As all attributes default to "false" according to the xml schema definition, the generated xml includes only those attributes that are set to true.
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag 'printOptions', str
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/protected_range.rb b/lib/axlsx/workbook/worksheet/protected_range.rb
index ac7d96f1..64a5795c 100644
--- a/lib/axlsx/workbook/worksheet/protected_range.rb
+++ b/lib/axlsx/workbook/worksheet/protected_range.rb
@@ -41,7 +41,7 @@ module Axlsx
     # @param [String] str if this string object is provided we append
     # our output to that object. Use this - it helps limit the number of
     # objects created during serialization
-    def to_xml_string(str = "")
+    def to_xml_string(str = +'')
       serialized_tag 'protectedRange', str
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/protected_ranges.rb b/lib/axlsx/workbook/worksheet/protected_ranges.rb
index 1d214d3b..f8ce82c5 100644
--- a/lib/axlsx/workbook/worksheet/protected_ranges.rb
+++ b/lib/axlsx/workbook/worksheet/protected_ranges.rb
@@ -28,7 +28,7 @@ module Axlsx
     # Serializes the protected ranges
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return if empty?
 
       str << '<protectedRanges>'
diff --git a/lib/axlsx/workbook/worksheet/rich_text.rb b/lib/axlsx/workbook/worksheet/rich_text.rb
index 13febb0a..e1a3ad4b 100644
--- a/lib/axlsx/workbook/worksheet/rich_text.rb
+++ b/lib/axlsx/workbook/worksheet/rich_text.rb
@@ -47,7 +47,7 @@ module Axlsx
     # renders the RichTextRuns in this collection
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       each { |run| run.to_xml_string(str) }
       str
     end
diff --git a/lib/axlsx/workbook/worksheet/rich_text_run.rb b/lib/axlsx/workbook/worksheet/rich_text_run.rb
index 9b61806c..ee988318 100644
--- a/lib/axlsx/workbook/worksheet/rich_text_run.rb
+++ b/lib/axlsx/workbook/worksheet/rich_text_run.rb
@@ -205,7 +205,7 @@ module Axlsx
     # Serializes the RichTextRun
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       valid = RichTextRun::INLINE_STYLES
       data = Hash[Axlsx.instance_values_for(self).map { |k, v| [k.to_sym, v] }]
       data = data.select { |key, value| valid.include?(key) && !value.nil? }
@@ -214,15 +214,15 @@ module Axlsx
       data.keys.each do |key|
         case key
         when :font_name
-          str << ('<rFont val="' << font_name << '"/>')
+          str << (+'<rFont val="' << font_name << '"/>')
         when :color
           str << data[key].to_xml_string
         else
-          str << ('<' << key.to_s << ' val="' << xml_value(data[key]) << '"/>')
+          str << (+'<' << key.to_s << ' val="' << xml_value(data[key]) << '"/>')
         end
       end
       clean_value = Axlsx::trust_input ? @value.to_s : ::CGI.escapeHTML(Axlsx::sanitize(@value.to_s))
-      str << ('</rPr><t>' << clean_value << '</t></r>')
+      str << (+'</rPr><t>' << clean_value << '</t></r>')
     end
 
     private
diff --git a/lib/axlsx/workbook/worksheet/row.rb b/lib/axlsx/workbook/worksheet/row.rb
index 50326aba..0c139164 100644
--- a/lib/axlsx/workbook/worksheet/row.rb
+++ b/lib/axlsx/workbook/worksheet/row.rb
@@ -88,10 +88,9 @@ module Axlsx
     # @param [Integer] r_index The row index, 0 based.
     # @param [String] str The string this rows xml will be appended to.
     # @return [String]
-    def to_xml_string(r_index, str = '')
+    def to_xml_string(r_index, str = +'')
       serialized_tag('row', str, :r => r_index + 1) do
-        tmp = '' # time / memory tradeoff, lots of calls to rubyzip costs more
-                 # time..
+        tmp = +'' # time / memory tradeoff, lots of calls to rubyzip costs more time..
         each_with_index { |cell, c_index| cell.to_xml_string(r_index, c_index, tmp) }
         str << tmp
       end
diff --git a/lib/axlsx/workbook/worksheet/row_breaks.rb b/lib/axlsx/workbook/worksheet/row_breaks.rb
index 6d1b17c8..3e945b78 100644
--- a/lib/axlsx/workbook/worksheet/row_breaks.rb
+++ b/lib/axlsx/workbook/worksheet/row_breaks.rb
@@ -23,10 +23,10 @@ module Axlsx
     # <brk id="7" max="16383" man="1"/>
     # <brk id="13" max="16383" man="1"/>
     # </rowBreaks>
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return if empty?
 
-      str << ('<rowBreaks count="' << self.size.to_s << '" manualBreakCount="' << self.size.to_s << '">')
+      str << (+'<rowBreaks count="' << self.size.to_s << '" manualBreakCount="' << self.size.to_s << '">')
       each { |brk| brk.to_xml_string(str) }
       str << '</rowBreaks>'
     end
diff --git a/lib/axlsx/workbook/worksheet/selection.rb b/lib/axlsx/workbook/worksheet/selection.rb
index 377c0b25..556651e3 100644
--- a/lib/axlsx/workbook/worksheet/selection.rb
+++ b/lib/axlsx/workbook/worksheet/selection.rb
@@ -94,7 +94,7 @@ module Axlsx
     # Serializes the data validation
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag 'selection', str
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/sheet_calc_pr.rb b/lib/axlsx/workbook/worksheet/sheet_calc_pr.rb
index bb78f92e..e85cbf2c 100644
--- a/lib/axlsx/workbook/worksheet/sheet_calc_pr.rb
+++ b/lib/axlsx/workbook/worksheet/sheet_calc_pr.rb
@@ -23,7 +23,7 @@ module Axlsx
     # @param [String] str the string to append this objects serialized
     # content to.
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << "<sheetCalcPr #{serialized_attributes}/>"
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/sheet_data.rb b/lib/axlsx/workbook/worksheet/sheet_data.rb
index 1531d5bf..611da1aa 100644
--- a/lib/axlsx/workbook/worksheet/sheet_data.rb
+++ b/lib/axlsx/workbook/worksheet/sheet_data.rb
@@ -16,7 +16,7 @@ module Axlsx
     # Serialize the sheet data
     # @param [String] str the string this objects serializaton will be concacted to.
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<sheetData>'
       worksheet.rows.each_with_index do |row, index|
         row.to_xml_string(index, str)
diff --git a/lib/axlsx/workbook/worksheet/sheet_format_pr.rb b/lib/axlsx/workbook/worksheet/sheet_format_pr.rb
index e9ef8f33..227b9037 100644
--- a/lib/axlsx/workbook/worksheet/sheet_format_pr.rb
+++ b/lib/axlsx/workbook/worksheet/sheet_format_pr.rb
@@ -48,7 +48,7 @@ module Axlsx
     # serializes this object to an xml string
     # @param [String] str The string this objects serialization will be appended to
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << "<sheetFormatPr #{serialized_attributes}/>"
     end
 
diff --git a/lib/axlsx/workbook/worksheet/sheet_pr.rb b/lib/axlsx/workbook/worksheet/sheet_pr.rb
index 3bbb88d4..f90e7d6c 100644
--- a/lib/axlsx/workbook/worksheet/sheet_pr.rb
+++ b/lib/axlsx/workbook/worksheet/sheet_pr.rb
@@ -50,7 +50,7 @@ module Axlsx
     # Serialize the object
     # @param [String] str serialized output will be appended to this object if provided.
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       update_properties
       str << "<sheetPr #{serialized_attributes}>"
       tab_color.to_xml_string(str, 'tabColor') if tab_color
diff --git a/lib/axlsx/workbook/worksheet/sheet_protection.rb b/lib/axlsx/workbook/worksheet/sheet_protection.rb
index 1b9aff14..faf34f97 100644
--- a/lib/axlsx/workbook/worksheet/sheet_protection.rb
+++ b/lib/axlsx/workbook/worksheet/sheet_protection.rb
@@ -75,7 +75,7 @@ module Axlsx
     # Serialize the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('sheetProtection', str)
     end
 
diff --git a/lib/axlsx/workbook/worksheet/sheet_view.rb b/lib/axlsx/workbook/worksheet/sheet_view.rb
index 38b9593a..d06715fe 100644
--- a/lib/axlsx/workbook/worksheet/sheet_view.rb
+++ b/lib/axlsx/workbook/worksheet/sheet_view.rb
@@ -192,7 +192,7 @@ module Axlsx
     # Serializes the data validation
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<sheetViews>'
       str << '<sheetView '
       serialized_attributes str
diff --git a/lib/axlsx/workbook/worksheet/table.rb b/lib/axlsx/workbook/worksheet/table.rb
index 70f27575..430ef489 100644
--- a/lib/axlsx/workbook/worksheet/table.rb
+++ b/lib/axlsx/workbook/worksheet/table.rb
@@ -73,14 +73,14 @@ module Axlsx
     # Serializes the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<?xml version="1.0" encoding="UTF-8"?>'
-      str << ('<table xmlns="' << XML_NS << '" id="' << (index + 1).to_s << '" name="' << @name << '" displayName="' << @name.gsub(/\s/, '_') << '" ')
-      str << ('ref="' << @ref << '" totalsRowShown="0">')
-      str << ('<autoFilter ref="' << @ref << '"/>')
-      str << ('<tableColumns count="' << header_cells.length.to_s << '">')
+      str << (+'<table xmlns="' << XML_NS << '" id="' << (index + 1).to_s << '" name="' << @name << '" displayName="' << @name.gsub(/\s/, '_') << '" ')
+      str << (+'ref="' << @ref << '" totalsRowShown="0">')
+      str << (+'<autoFilter ref="' << @ref << '"/>')
+      str << (+'<tableColumns count="' << header_cells.length.to_s << '">')
       header_cells.each_with_index do |cell, index|
-        str << ('<tableColumn id ="' << (index + 1).to_s << '" name="' << cell.clean_value << '"/>')
+        str << (+'<tableColumn id ="' << (index + 1).to_s << '" name="' << cell.clean_value << '"/>')
       end
       str << '</tableColumns>'
       table_style_info.to_xml_string(str)
diff --git a/lib/axlsx/workbook/worksheet/table_style_info.rb b/lib/axlsx/workbook/worksheet/table_style_info.rb
index 1af61d9f..3793d488 100644
--- a/lib/axlsx/workbook/worksheet/table_style_info.rb
+++ b/lib/axlsx/workbook/worksheet/table_style_info.rb
@@ -43,7 +43,7 @@ module Axlsx
 
     # seralizes this object to an xml string
     # @param [String] str the string to contact this objects serialization to.
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       serialized_tag('tableStyleInfo', str)
     end
   end
diff --git a/lib/axlsx/workbook/worksheet/tables.rb b/lib/axlsx/workbook/worksheet/tables.rb
index 41d3c658..121a0d7f 100644
--- a/lib/axlsx/workbook/worksheet/tables.rb
+++ b/lib/axlsx/workbook/worksheet/tables.rb
@@ -25,7 +25,7 @@ module Axlsx
     # renders the tables xml
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = "")
+    def to_xml_string(str = +'')
       return if empty?
 
       str << "<tableParts count='#{size}'>"
diff --git a/lib/axlsx/workbook/worksheet/worksheet.rb b/lib/axlsx/workbook/worksheet/worksheet.rb
index 1e8bf2e3..0a44027f 100644
--- a/lib/axlsx/workbook/worksheet/worksheet.rb
+++ b/lib/axlsx/workbook/worksheet/worksheet.rb
@@ -628,18 +628,18 @@ module Axlsx
     end
 
     # Returns a sheet node serialization for this sheet in the workbook.
-    def to_sheet_node_xml_string(str = '')
+    def to_sheet_node_xml_string(str = +'')
       add_autofilter_defined_name_to_workbook
       str << '<sheet '
       serialized_attributes str
-      str << ('name="' << name << '" ')
-      str << ('r:id="' << rId << '"></sheet>')
+      str << (+'name="' << name << '" ')
+      str << (+'r:id="' << rId << '"></sheet>')
     end
 
     # Serializes the worksheet object to an xml string
     # This intentionally does not use nokogiri for performance reasons
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       add_autofilter_defined_name_to_workbook
       auto_filter.apply if auto_filter.range
       str << '<?xml version="1.0" encoding="UTF-8"?>'
diff --git a/lib/axlsx/workbook/worksheet/worksheet_comments.rb b/lib/axlsx/workbook/worksheet/worksheet_comments.rb
index 0f0355e7..c5f5ad13 100644
--- a/lib/axlsx/workbook/worksheet/worksheet_comments.rb
+++ b/lib/axlsx/workbook/worksheet/worksheet_comments.rb
@@ -51,7 +51,7 @@ module Axlsx
     # Seraalize the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return unless has_comments?
 
       str << "<legacyDrawing r:id='#{drawing_rId}' />"
diff --git a/lib/axlsx/workbook/worksheet/worksheet_drawing.rb b/lib/axlsx/workbook/worksheet/worksheet_drawing.rb
index ce7edc7c..18c7189a 100644
--- a/lib/axlsx/workbook/worksheet/worksheet_drawing.rb
+++ b/lib/axlsx/workbook/worksheet/worksheet_drawing.rb
@@ -52,7 +52,7 @@ module Axlsx
 
     # Serialize the drawing for the worksheet
     # @param [String] str
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return unless has_drawing?
 
       str << "<drawing r:id='#{relationship.Id}'/>"
diff --git a/lib/axlsx/workbook/worksheet/worksheet_hyperlink.rb b/lib/axlsx/workbook/worksheet/worksheet_hyperlink.rb
index f375582b..c4cb8983 100644
--- a/lib/axlsx/workbook/worksheet/worksheet_hyperlink.rb
+++ b/lib/axlsx/workbook/worksheet/worksheet_hyperlink.rb
@@ -58,7 +58,7 @@ module Axlsx
     # Seralize the object
     # @param [String] str
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       str << '<hyperlink '
       serialized_attributes str, location_or_id
       str << '/>'
diff --git a/lib/axlsx/workbook/worksheet/worksheet_hyperlinks.rb b/lib/axlsx/workbook/worksheet/worksheet_hyperlinks.rb
index a5b1a512..f19630f8 100644
--- a/lib/axlsx/workbook/worksheet/worksheet_hyperlinks.rb
+++ b/lib/axlsx/workbook/worksheet/worksheet_hyperlinks.rb
@@ -29,7 +29,7 @@ module Axlsx
 
     # seralize the collection of hyperlinks
     # @return [String]
-    def to_xml_string(str = '')
+    def to_xml_string(str = +'')
       return if empty?
 
       str << '<hyperlinks>'
author	Paul Kmiec <[email protected]>	2023-05-04 16:31:08 -0700
committer	Paul Kmiec <[email protected]>	2023-05-04 18:02:55 -0700
commit	fabe8cb2571c8865270247ca78ddc01d493a9ee1 (patch)
tree	27fe0b0df8c9bec8f746d92a0d128cad197027b2 /lib/axlsx
parent	fef93ec8ae2caf8a3f8310dbf8101c103e5905e4 (diff)
download	caxlsx-fabe8cb2571c8865270247ca78ddc01d493a9ee1.tar.gz caxlsx-fabe8cb2571c8865270247ca78ddc01d493a9ee1.zip