Use safe_concat instead of trying to html_safe! what might be a frozen string

actionpack/lib/action_controller/caching/fragments.rb

@@ -36,8 +36,8 @@ def fragment_cache_key(key)
 
       def fragment_for(buffer, name = {}, options = nil, &block) #:nodoc:
         if perform_caching
-          if fragment_exist?(name,options)
-            buffer.concat(read_fragment(name, options).html_safe!)
+          if fragment_exist?(name, options)
+            buffer.safe_concat(read_fragment(name, options))
           else
             pos = buffer.length
             block.call