Return a json dictionary representing this model.

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