Only mark the sliced buffer as safe, if it was safe to begin with.

actionpack/lib/action_view/helpers/cache_helper.rb

@@ -51,8 +51,9 @@ def fragment_for(name = {}, options = nil, &block) #:nodoc:
           # This dance is needed because Builder can't use capture
           pos = output_buffer.length
           yield
+          output_safe = output_buffer.html_safe?
           fragment = output_buffer.slice!(pos..-1)
-          if output_buffer.kind_of?(ActiveSupport::SafeBuffer)
+          if output_safe
             self.output_buffer = output_buffer.html_safe
           end
           controller.write_fragment(name, fragment, options)