Support frozen string literals (#105)

This is to avoid deprecation warnings with the forthcoming change in
Ruby 3.4 where modifying a string literal will cause a deprecation
warning by default.
ruby/ruby@12be40a

Co-authored-by: Johannes Opper <[email protected]>

lib/reverse_markdown/converters/base.rb

@@ -2,7 +2,7 @@ module ReverseMarkdown
   module Converters
     class Base
       def treat_children(node, state)
-        node.children.inject('') do |memo, child|
+        node.children.inject(+'') do |memo, child|
           memo << treat(child, state)
         end
       end

lib/reverse_markdown/converters/blockquote.rb

@@ -4,7 +4,7 @@ class Blockquote < Base
       def convert(node, state = {})
         content = treat_children(node, state).strip
         content = ReverseMarkdown.cleaner.remove_newlines(content)
-        "\n\n> " << content.lines.to_a.join('> ') << "\n\n"
+        +"\n\n> " << content.lines.to_a.join('> ') << "\n\n"
       end
     end
 

lib/reverse_markdown/converters/div.rb

@@ -2,7 +2,7 @@ module ReverseMarkdown
   module Converters
     class Div < Base
       def convert(node, state = {})
-        "\n" << treat_children(node, state) << "\n"
+        +"\n" << treat_children(node, state) << "\n"
       end
     end
 

lib/reverse_markdown/converters/figcaption.rb

@@ -5,7 +5,7 @@ def convert(node, state = {})
         if node.text.strip.empty?
           ""
         else
-          "\n" << "_#{node.text.strip}_" << "\n"
+          +"\n" << "_#{node.text.strip}_" << "\n"
         end
       end
     end

lib/reverse_markdown/converters/ol.rb

@@ -3,7 +3,7 @@ module Converters
     class Ol < Base
       def convert(node, state = {})
         ol_count = state.fetch(:ol_count, 0) + 1
-        "\n" << treat_children(node, state.merge(ol_count: ol_count)) << "\n"
+        +"\n" << treat_children(node, state.merge(ol_count: ol_count)) << "\n"
       end
     end
 

lib/reverse_markdown/converters/p.rb

@@ -2,7 +2,7 @@ module ReverseMarkdown
   module Converters
     class P < Base
       def convert(node, state = {})
-        "\n\n" << treat_children(node, state).strip << "\n\n"
+        +"\n\n" << treat_children(node, state).strip << "\n\n"
       end
     end
 

lib/reverse_markdown/converters/pre.rb

@@ -4,9 +4,9 @@ class Pre < Base
       def convert(node, state = {})
         content = treat_children(node, state)
         if ReverseMarkdown.config.github_flavored
-          "\n```#{language(node)}\n" << content.strip << "\n```\n"
+          +"\n```#{language(node)}\n" << content.strip << "\n```\n"
         else
-          "\n\n    " << content.lines.to_a.join("    ") << "\n\n"
+          +"\n\n    " << content.lines.to_a.join("    ") << "\n\n"
         end
       end
 

lib/reverse_markdown/converters/table.rb

@@ -2,7 +2,7 @@ module ReverseMarkdown
   module Converters
     class Table < Base
       def convert(node, state = {})
-        "\n\n" << treat_children(node, state) << "\n"
+        +"\n\n" << treat_children(node, state) << "\n"
       end
     end