Return a json dictionary representing this model.

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