'Verify that the stub sent a POST request back to us
with the expected data.
`callback_url` is the URL we expect the stub to POST to
`expected_header` is the header (a string) we expect to receive with the grade.
`expected_body` is the content (a string) we expect to receive with the grade.
Raises an `AssertionError` if the check fails.'

def _check_grade_response(self, callback_url, expected_header, expected_body):

    expected_callback_dict = {'xqueue_header': expected_header, 'xqueue_body': expected_body}
    self.post.assert_called_with(callback_url, data=expected_callback_dict)
