Encrypt #to_key so that generated HTML does not inadvertently expose unencrypted ID

This makes sure the `form_for` helper does not expose the unencrypted resource ID which otherwise will happen if to_key is not overridden.

Some background:

https://github.com/rails/rails/blob/3-2-13/actionpack/lib/action_view/helpers/form_helper.rb#L184-L201

https://github.com/rails/rails/blob/3-2-13/actionpack/lib/action_view/helpers/form_helper.rb#L391

http://apidock.com/rails/ActionController/RecordIdentifier/dom_id

http://apidock.com/rails/ActionController/RecordIdentifier/record_key_for_dom_id