Return a json dictionary representing this model.

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