Submit a single `multipart/mixed` request with deferred requests.

        :rtype: list of tuples
        :returns: one ``(headers, payload)`` tuple per deferred request.

def finish(self):
        """Submit a single `multipart/mixed` request with deferred requests.

        :rtype: list of tuples
        :returns: one ``(headers, payload)`` tuple per deferred request.
        """
        headers, body = self._prepare_batch_request()

        url = "%s/batch/storage/v1" % self.API_BASE_URL

        # Use the private ``_base_connection`` rather than the property
        # ``_connection``, since the property may be this
        # current batch.
        response = self._client._base_connection._make_request(
            "POST", url, data=body, headers=headers
        )
        responses = list(_unpack_batch_response(response))
        self._finish_futures(responses)
        return responses