'Submit answers to a question.
Responses is a dict mapping problem ids to answers:
{\'2_1\': \'Correct\', \'2_2\': \'Incorrect\'}'

def submit_question_answer(self, problem_url_name, responses):

    problem_location = self.problem_location(problem_url_name)
    modx_url = self.modx_url(problem_location, 'problem_check')
    answer_key_prefix = 'input_{}_'.format(problem_location.html_id())
    response_dict = {(answer_key_prefix + k): v for (k, v) in responses.items()}
    resp = self.client.post(modx_url, response_dict)
    return resp
