Add performance rubocop and fixes

test/integration/drop_test.rb

@@ -33,7 +33,7 @@ class ProductDrop < Liquid::Drop
 
   class CatchallDrop < Liquid::Drop
     def liquid_method_missing(method)
-      'catchall_method: '.dup << method.to_s
+      +'catchall_method: ' << method.to_s
     end
   end
 
@@ -50,7 +50,7 @@ class ProductDrop < Liquid::Drop
   end
 
   def user_input
-    'foo'.dup.taint
+    (+'foo').taint
   end
 
   protected

test/unit/block_unit_test.rb

@@ -63,7 +63,7 @@ class BlockUnitTest < Minitest::Test
 
       assert_equal 'hello', template.render
 
-      buf = ''.dup
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'hello', output
       assert_equal 'hello', buf
@@ -81,7 +81,7 @@ class BlockUnitTest < Minitest::Test
 
       assert_equal 'foohellobar', template.render
 
-      buf = ''.dup
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'foohellobar', output
       assert_equal 'foohellobar', buf

test/unit/tag_unit_test.rb

@@ -33,7 +33,7 @@ class TagUnitTest < Minitest::Test
 
       assert_equal 'hello', template.render
 
-      buf = ''.dup
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'hello', output
       assert_equal 'hello', buf
@@ -51,7 +51,7 @@ class TagUnitTest < Minitest::Test
 
       assert_equal 'foohellobar', template.render
 
-      buf = ''.dup
+      buf = +''
       output = template.render({}, output: buf)
       assert_equal 'foohellobar', output
       assert_equal 'foohellobar', buf