removes choiceresponse wiping after clicking Show Answer

[auraz]

From the ticket: "If you click "show answers", then click "check", your own answers are wiped out (presumably because the radio buttons are disabled). I think this is a general issue with multiple choice questions. The only way around this is to click "show answers", navigate to another page, go back to the original page, then hit "check" again."

Support is receiving a lot of mail about this, and numerous professors have complained.

Description in https://edx-wiki.atlassian.net/browse/BLD-128, complaints in https://edx.lighthouseapp.com/projects/101932/tickets/219-Multiple-Choice-clears-answers

Sample problems (for easy testing) added to https://github.com/MITx/content-demos repo in /Testing/Testing:Choiceresponsebug/ sequence.

@sarina @arjun810 please review.

[arjun810]

I can't access JIRA, so I can't comment there, but I don't think it's necessary. I think (but I'm not 100% sure) that the reason we disabled it before was that we used to use the checkboxes themselves to show which answers were correct, but then later were able to do so using the green check / red x.

[sarina]

@auraz you should include the text from JIRA tickets, along with the link to JIRA, in each PR. Because we are open source, but JIRA is not, all PRs should be able to stand on their own (but include the JIRA reference for those who do have access).

[sarina]

@arjun810

I think (but I'm not 100% sure) that the reason we disabled it before was that we used to use the checkboxes themselves to show which answers were correct, but then later were able to do so using the green check / red x.

I think that makes sense. However at this point, multiplechoiceresponse doesn't behave like any other problem, which is not a good thing. For example, string response, code response, formula response, and
option response, if you click "Show Answer", you are allowed to change your
answer while the answer is displayed (so yes, this allows you to click
"Show Answer" then just copy all the right answers into the boxes). However for multiple choice response, clicking "Show Answer" greys out the radio/check buttons and disallows you from changing your answer in response
to the new answer being displayed. This UX is inconsistent.

(just trying to share details of the JIRA/lighthouse ticket with you so you're up to speed on the PR)

[sarina]

👍 looks good to me, and behavior now matches behavior of other types of problems.