Bug: removing network connection should not result in wrong answers being marked 100% correct

[sladen]

To reproduce:

1. Open a test/question/practice
2. Remove the network connection
3. Fill in any, no, or some answer (eg. "foobar")
4. Click [Continue]
5. Observe that the answer is marked Correct
6. Repeat for all questions in the test
7. Observe that the score is 100%
8. Click on [Review]
9. Click on an individual answer
10. Observe that this shows: 'Your response: "foobar" Correct response: "" '

What happens:

1. It would appear that if a callout fails, a null string is returned
2. Testing against this null string returns success

What should happen:

1. An incorrect answer should not be marked correct
2. Ideally answers should be held and compared when network returns if a local copy is not available

At least one other user has observed this behaviour back in c. October 2014:

https://www.duolingo.com/comment/1298532#comment-4467005

[kiddo-depido]

I sometimes experienced that in the mobile version, but it was not intended, it happened because my WiFi is horrible xD

I noticed it some lessons later... Before: "Oh, I must be Goethe"... haha