Write strings to fragment cache, not outputbuffers

actionpack/lib/action_controller/caching/fragments.rb

@@ -41,7 +41,9 @@ def fragment_for(buffer, name = {}, options = nil, &block) #:nodoc:
           else
             pos = buffer.length
             block.call
-            write_fragment(name, buffer[pos..-1], options)
+            content = buffer[pos..-1]
+            content = content.as_str if content.respond_to?(:as_str)
+            write_fragment(name, content, options)
           end
         else
           block.call

activesupport/lib/active_support/core_ext/string/output_safety.rb

@@ -89,8 +89,12 @@ def to_s
       self
     end
 
+    def as_str
+      ''.replace(self)
+    end
+
     def to_yaml
-      "".replace(self).to_yaml
+      as_str.to_yaml
     end
   end
 end