Fix a bug with embedded documents

Hi @jnicklas,

I found a bug when I was upgrading LocomotiveCMS to mongoid 3.x. Actually, mongoid prevents carriewave to attach a file to a embedded document owned by an existing document. It only occurs if the embedded document didn't have a attached file when the parent document was created. A little bit more of explanation here: https://github.com/jnicklas/carrierwave-mongoid/pull/64#issuecomment-17389921

To be honest, I'm not satisfied by my "patch" but anyway, it solves the bug.
Thanks !

  Didier