Return a deferred.

def save(self, idempotency_key=None):
        """Return a deferred."""
        updated_params = self.serialize(None)
        headers = populate_headers(idempotency_key)

        if not updated_params:
            util.logger.debug("Trying to save already saved object %r", self)
            return defer.succeed(self)

        d = self.request('post', self.instance_url(), updated_params, headers)
        return d.addCallback(self.refresh_from).addCallback(lambda _: self)