Mongokit lazy_migration is not working when unsetting fields

[AlmogCohen]

I found exactly the code responsible for the bug.
in migration.py

               doc.collection.update(self.target, self.update, multi=False, safe=safe)
                # reload
                try:
                    doc.update(doc.collection.get_from_id(doc['_id']))
                except:
                    raise OperationFailure('Can not reload an unsaved document. '
                                           '%s is not found in the database' % doc['_id'])

The local document is updated with update and therefore removed fields just stay inside the document and cause the same exception for an extra field