Unset parent_row fields when processing before_save for embedded objects

The current code does not allow fields to be delete from the parent row during the before_save callback of an embedded objects. Or rather fields can be deleted from the hash, but they are not "unset" in the database.
Although maybe a rare case, in some 1-to-1 relationships it is useful, when migrating from SQL to Mongo, to push some fields of the parent object to the embedded one.
Together with the code, I added specs to test the new functionality and a new cucumber scenario to the process feature which test the new functionality.

regards,
marco