Add performance rubocop and fixes

lib/liquid/block_body.rb

@@ -65,7 +65,7 @@ module Liquid
     end
 
     def render(context)
-      render_to_output_buffer(context, ''.dup)
+      render_to_output_buffer(context, +'')
     end
 
     def render_to_output_buffer(context, output)
@@ -81,7 +81,7 @@ module Liquid
         when Variable
           render_node(context, output, node)
         when Block
-          render_node(context, node.blank? ? ''.dup : output, node)
+          render_node(context, node.blank? ? +'' : output, node)
           break if context.interrupt? # might have happened in a for-block
         when Continue, Break
           # If we get an Interrupt that means the block must stop processing. An

lib/liquid/errors.rb

@@ -7,7 +7,7 @@ module Liquid
     attr_accessor :markup_context
 
     def to_s(with_prefix = true)
-      str = ''.dup
+      str = +''
       str << message_prefix if with_prefix
       str << super()
 
@@ -22,7 +22,7 @@ module Liquid
     private
 
     def message_prefix
-      str = ''.dup
+      str = +''
       str << if is_a?(SyntaxError)
         'Liquid syntax error'
       else

lib/liquid/parser.rb

@@ -70,7 +70,7 @@ module Liquid
     end
 
     def argument
-      str = ''.dup
+      str = +''
       # might be a keyword argument (identifier: expression)
       str << consume << consume << ' ' if look(:id) && look(:colon, 1)
 

lib/liquid/tags/ifchanged.rb

@@ -4,7 +4,7 @@ module Liquid
   class Ifchanged < Block
     def render_to_output_buffer(context, output)
       context.stack do
-        block_output = ''.dup
+        block_output = +''
         super(context, block_output)
 
         if block_output != context.registers[:ifchanged]

lib/liquid/tags/raw.rb

@@ -12,7 +12,7 @@ module Liquid
     end
 
     def parse(tokens)
-      @body = ''.dup
+      @body = +''
       while (token = tokens.shift)
         if token =~ FULL_TOKEN_POSSIBLY_INVALID
           @body << Regexp.last_match(1) if Regexp.last_match(1) != ''

lib/liquid/template.rb

@@ -204,7 +204,7 @@ module Liquid
         # render the nodelist.
         # for performance reasons we get an array back here. join will make a string out of it.
         with_profiling(context) do
-          @root.render_to_output_buffer(context, output || ''.dup)
+          @root.render_to_output_buffer(context, output || +'')
         end
       rescue Liquid::MemoryError => e
         context.handle_error(e)