Proper warning support

lib/liquid/tags/for.rb

@@ -58,6 +58,7 @@ module Liquid
     end
 
     def render(context)
+      context.errors.concat(@warnings) if @warnings
       context.registers[:for] ||= Hash.new(0)
 
       collection = context[@collection_name]

lib/liquid/tags/if.rb

@@ -29,7 +29,7 @@ module Liquid
     end
 
     def render(context)
-      context.errors += @warnings if @warnings
+      context.errors.concat(@warnings) if @warnings
       context.stack do
         @blocks.each do |block|
           if block.evaluate(context)