def METHOD_NAME(self, requests_post):
    url = 'http://callback-target.com'
    headers = {'Authorization': 'token 654321'}
    callback = self.build.callbacks.create(url=url, event=self.event, headers=json.dumps(headers))
    callback.dispatch()
    self.assertTrue(requests_post.called)
    self.assertTrue(callback.is_sent)
    requests_post.assert_called_with(url, headers=headers)