Return a deferred.

def refund(self, idempotency_key=None, **params):
        """Return a deferred."""
        headers = populate_headers(idempotency_key)
        url = self.instance_url() + '/refund'
        d = self.request('post', url, params, headers)
        return d.addCallback(self.refresh_from).addCallback(lambda _: self)