Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'status') and self.status is not None:
            _dict['status'] = self.status
        if hasattr(self, 'last_updated') and self.last_updated is not None:
            _dict['last_updated'] = datetime_to_string(self.last_updated)
        return _dict