Fix reverting unsaved changes for entities with custom (de)serialization

The issue is (IMHO) that `self._rowValuesInDatabase[fieldName]` yields for `BLOB` columns the unserialized `NSData` object. If however an entity chooses to deserialize this to e.g. `UIImage` the deserialization was not applied when reverting all unsaved changes. The method `revertUnsavedChangeToFieldName:` did so correctly. The following change simply calls `revertUnsavedChangeToFieldName:` for all unsaved columns in `revertUnsavedChanges `.