HTTP caching in FoldersController#show is based on last-modified, not etag
Added folders_updated_at attribute to User model.

It defaults to the current date-time when saving a User instance.

It will help with implementing HTTP caching for the FoldersController#index action.
When a folder is created or destroyed, the folders_updated_at attribute of its User is set to the current date/time.
Added HTTP caching support to FoldersController#index action
Do not send a parameter with the current time when requesting folders via Ajax.

This kept the client from leveraging HTTP caching.