'Post a submission to the stub XQueue implementation.
`callback_url` is the URL at which we expect to receive a grade response
`lms_key` is the authentication key sent in the header
`queue_name` is the name of the queue in which to send put the submission
`xqueue_body` is the content of the submission
Returns the header (a string) we send with the submission, which can
be used to validate the response we receive from the stub.'

def _post_submission(self, callback_url, lms_key, queue_name, xqueue_body):

    grade_request = {'xqueue_header': json.dumps({'lms_callback_url': callback_url, 'lms_key': 'test_queuekey', 'queue_name': 'test_queue'}), 'xqueue_body': xqueue_body}
    resp = requests.post(self.url, data=grade_request)
    self.assertEqual(resp.status_code, 200)
    return grade_request['xqueue_header']
