'Request account creation with the given params and assert that the
response properly indicates an error with the given field and value'

def assert_error(self, params, expected_field, expected_value):

    response = self.client.post(self.url, params)
    self.assertEqual(response.status_code, 400)
    response_data = json.loads(response.content)
    self.assertFalse(response_data['success'])
    self.assertEqual(response_data['field'], expected_field)
    self.assertEqual(response_data['value'], expected_value)
