Fix StringConcatenation offenses (non-production)

Prefer interpolation over concatenation

```
Comparison ("String#{'String'}" vs 'String' + 'String'):
       interpolation: 11821321.0 i/s
       concatenation:  8849491.7 i/s - 1.34x  (± 0.00) slower
```

11 files changed, 23 insertions, 16 deletions

diff --git a/.rubocop_todo.yml b/.rubocop_todo.yml
index a0260f2e..82fc6a22 100644
--- a/.rubocop_todo.yml
+++ b/.rubocop_todo.yml
@@ -423,7 +423,14 @@ Style/StringChars:
 # This cop supports unsafe autocorrection (--autocorrect-all).
 # Configuration parameters: Mode.
 Style/StringConcatenation:
-  Enabled: false
+  Exclude:
+    - 'lib/axlsx/drawing/area_series.rb'
+    - 'lib/axlsx/drawing/line_series.rb'
+    - 'lib/axlsx/drawing/pie_series.rb'
+    - 'lib/axlsx/drawing/scatter_series.rb'
+    - 'lib/axlsx/stylesheet/color.rb'
+    - 'lib/axlsx/util/constants.rb'
+    - 'lib/axlsx/workbook/worksheet/worksheet.rb'
 
 # This cop supports safe autocorrection (--autocorrect).
 # Configuration parameters: EnforcedStyle, ConsistentQuotesInMultiline.
diff --git a/Rakefile b/Rakefile
index 8125959f..28712c0d 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,13 +1,13 @@
 # frozen_string_literal: true
 
-require File.expand_path(File.dirname(__FILE__) + '/lib/axlsx/version.rb')
+require File.expand_path("#{File.dirname(__FILE__)}/lib/axlsx/version.rb")
 
 task build: :gendoc do
   system "gem build axlsx.gemspec"
 end
 
 task :benchmark do
-  require File.expand_path(File.dirname(__FILE__) + '/test/benchmark.rb')
+  require File.expand_path("#{File.dirname(__FILE__)}/test/benchmark.rb")
 end
 
 task :gendoc do
diff --git a/test/doc_props/tc_app.rb b/test/doc_props/tc_app.rb
index 82e33f12..c066373c 100644
--- a/test/doc_props/tc_app.rb
+++ b/test/doc_props/tc_app.rb
@@ -41,6 +41,6 @@ class TestApp < Test::Unit::TestCase
       errors << error
     end
 
-    assert_equal(0, errors.size, "app.xml invalid" + errors.map(&:message).to_s)
+    assert_equal(0, errors.size, "app.xml invalid#{errors.map(&:message)}")
   end
 end
diff --git a/test/doc_props/tc_core.rb b/test/doc_props/tc_core.rb
index bfacab45..5a04a615 100644
--- a/test/doc_props/tc_core.rb
+++ b/test/doc_props/tc_core.rb
@@ -18,7 +18,7 @@ class TestCore < Test::Unit::TestCase
       errors << error
     end
 
-    assert_equal(0, errors.size, "core.xml Invalid" + errors.map(&:message).to_s)
+    assert_equal(0, errors.size, "core.xml Invalid#{errors.map(&:message)}")
   end
 
   def test_populates_created
diff --git a/test/drawing/tc_drawing.rb b/test/drawing/tc_drawing.rb
index d7e3e8df..d48ae604 100644
--- a/test/drawing/tc_drawing.rb
+++ b/test/drawing/tc_drawing.rb
@@ -26,7 +26,7 @@ class TestDrawing < Test::Unit::TestCase
   end
 
   def test_add_image
-    src = File.dirname(__FILE__) + "/../fixtures/image1.jpeg"
+    src = "#{File.dirname(__FILE__)}/../fixtures/image1.jpeg"
     image = @ws.add_image(image_src: src, start_at: [0, 0], width: 600, height: 400)
 
     assert(@ws.drawing.anchors.last.is_a?(Axlsx::OneCellAnchor))
@@ -36,7 +36,7 @@ class TestDrawing < Test::Unit::TestCase
   end
 
   def test_add_two_cell_anchor_image
-    src = File.dirname(__FILE__) + "/../fixtures/image1.jpeg"
+    src = "#{File.dirname(__FILE__)}/../fixtures/image1.jpeg"
     image = @ws.add_image(image_src: src, start_at: [0, 0], end_at: [15, 0])
 
     assert(@ws.drawing.anchors.last.is_a?(Axlsx::TwoCellAnchor))
diff --git a/test/drawing/tc_hyperlink.rb b/test/drawing/tc_hyperlink.rb
index c3cffb1c..5a1f5d48 100644
--- a/test/drawing/tc_hyperlink.rb
+++ b/test/drawing/tc_hyperlink.rb
@@ -6,7 +6,7 @@ class TestHyperlink < Test::Unit::TestCase
   def setup
     @p = Axlsx::Package.new
     ws = @p.workbook.add_worksheet
-    @test_img = File.dirname(__FILE__) + "/../fixtures/image1.jpeg"
+    @test_img = "#{File.dirname(__FILE__)}/../fixtures/image1.jpeg"
     @image = ws.add_image image_src: @test_img, hyperlink: "http://axlsx.blogspot.com"
     @hyperlink = @image.hyperlink
   end
diff --git a/test/drawing/tc_one_cell_anchor.rb b/test/drawing/tc_one_cell_anchor.rb
index e3eb8d90..5015d123 100644
--- a/test/drawing/tc_one_cell_anchor.rb
+++ b/test/drawing/tc_one_cell_anchor.rb
@@ -6,7 +6,7 @@ class TestOneCellAnchor < Test::Unit::TestCase
   def setup
     @p = Axlsx::Package.new
     @ws = @p.workbook.add_worksheet
-    @test_img = File.dirname(__FILE__) + "/../fixtures/image1.jpeg"
+    @test_img = "#{File.dirname(__FILE__)}/../fixtures/image1.jpeg"
     @image = @ws.add_image image_src: @test_img
     @anchor = @image.anchor
   end
diff --git a/test/drawing/tc_pic.rb b/test/drawing/tc_pic.rb
index dc0f32da..f4961d4c 100644
--- a/test/drawing/tc_pic.rb
+++ b/test/drawing/tc_pic.rb
@@ -9,10 +9,10 @@ class TestPic < Test::Unit::TestCase
 
     @p = Axlsx::Package.new
     ws = @p.workbook.add_worksheet
-    @test_img = @test_img_jpg = File.dirname(__FILE__) + "/../fixtures/image1.jpeg"
-    @test_img_png =  File.dirname(__FILE__) + "/../fixtures/image1.png"
-    @test_img_gif =  File.dirname(__FILE__) + "/../fixtures/image1.gif"
-    @test_img_fake = File.dirname(__FILE__) + "/../fixtures/image1_fake.jpg"
+    @test_img = @test_img_jpg = "#{File.dirname(__FILE__)}/../fixtures/image1.jpeg"
+    @test_img_png =  "#{File.dirname(__FILE__)}/../fixtures/image1.png"
+    @test_img_gif =  "#{File.dirname(__FILE__)}/../fixtures/image1.gif"
+    @test_img_fake = "#{File.dirname(__FILE__)}/../fixtures/image1_fake.jpg"
     @test_img_remote_png = "https://example.com/sample-image.png"
     @test_img_remote_fake = "invalid_URI"
     @image = ws.add_image image_src: @test_img, hyperlink: 'https://github.com/randym', tooltip: "What's up doc?", opacity: 5
diff --git a/test/util/tc_mime_type_utils.rb b/test/util/tc_mime_type_utils.rb
index fc7d9d79..16ba45cd 100644
--- a/test/util/tc_mime_type_utils.rb
+++ b/test/util/tc_mime_type_utils.rb
@@ -7,7 +7,7 @@ class TestMimeTypeUtils < Test::Unit::TestCase
     stub_request(:get, 'https://example.com/sample-image.png')
       .to_return(body: File.new('examples/sample.png'), status: 200)
 
-    @test_img = File.dirname(__FILE__) + "/../fixtures/image1.jpeg"
+    @test_img = "#{File.dirname(__FILE__)}/../fixtures/image1.jpeg"
     @test_img_url = "https://example.com/sample-image.png"
   end
 
diff --git a/test/workbook/tc_shared_strings_table.rb b/test/workbook/tc_shared_strings_table.rb
index 2edf5f4a..4ac8b112 100644
--- a/test/workbook/tc_shared_strings_table.rb
+++ b/test/workbook/tc_shared_strings_table.rb
@@ -45,7 +45,7 @@ class TestSharedStringsTable < Test::Unit::TestCase
       errors << error
     end
 
-    assert_equal(0, errors.size, "sharedStirngs.xml Invalid" + errors.map(&:message).to_s)
+    assert_equal(0, errors.size, "sharedStirngs.xml Invalid#{errors.map(&:message)}")
   end
 
   def test_remove_control_characters_in_xml_serialization
diff --git a/test/workbook/worksheet/tc_rich_text_run.rb b/test/workbook/worksheet/tc_rich_text_run.rb
index 6274dda8..be05d818 100644
--- a/test/workbook/worksheet/tc_rich_text_run.rb
+++ b/test/workbook/worksheet/tc_rich_text_run.rb
@@ -151,7 +151,7 @@ class RichTextRun < Test::Unit::TestCase
 
   def test_multiline_autowidth
     wrap = @p.workbook.styles.add_style({ alignment: { wrap_text: true } })
-    awtr = Axlsx::RichTextRun.new('I\'m bold' + "\n", b: true)
+    awtr = Axlsx::RichTextRun.new("I'm bold\n", b: true)
     rt = Axlsx::RichText.new
     rt.runs << awtr
     @ws.add_row [rt], style: wrap