Fix removing uploaded files in embedded documents.

Sending `remove_file = "1"` to an embedded document does not work: Mongoid won’t consider the document dirty and so `save` won’t do anything. (Note that this bug only occurs if there are no other changes to the document.)

I'm not 100% sure how this relates to https://github.com/carrierwaveuploader/carrierwave-mongoid/pull/95 – calling `uploader_column_will_change!` seems to suffice in this case.