Return a json dictionary representing this model.

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