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

Without cache key expanding, following code would generate wrong cache key

```ruby
json.cache! [current_user, current_user.issues] do #=> /user/1-20120806214154/<AR::Associations::CollectionProxy>/7a1156131a6928cb0026877f8b749ac9
  # blabla
end
```

With my patch, jbuilder can generate right cache key.