Fix tests / code to work with frozen string literals

1 files changed, 5 insertions, 5 deletions

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