Return a json dictionary representing this model.

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