fix cache key generation for complex object

In Rails > 4, cache key would be expand fragment name with digest.

https://github.com/rails/rails/blob/4-0-stable/actionpack/lib/action_view/helpers/cache_helper.rb#L115
https://github.com/rails/rails/blob/4-0-stable/actionpack/lib/action_controller/caching/fragments.rb#L21-23
Merge pull request #201 from kaichen/master

fix cache key generation for complex object