Return a json dictionary representing this model.

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