Multi-choice answers checkbox does not preserve state when navigating quiz

The Problem

When we turn on backwards navigation and we have multi-choice questions that support multiple answers the checkbox states don't get preserved.

Steps to reproduce:

Step 1: Create the Quiz
1. Create a new quiz
2. Click the “Backwards” navigation checkbox
3. Add three questions each with their own answers.
4. For your questions give them the following settings:

• Question 1: Accept multiple answers
• Question 2: Do not accept multiple answers
• Question 3: Accept multiple answers.

Step 2: Run the quiz
1. Start the quiz
2. On Question 1 choose two checkboxes
3. On question 2 choose one radio button.
4. On question 3 click back
- Note that on question 2 the radio button is checked
5. On question 2 click back
- Note that the checkboxes in question 1 are not checked. (these should be checked)

Proposed solution:

In multichoice.classes.inc::getAnsweringForm().

if (is_array($response->getResponse())) {
    $element['#default_value'] = $response->getResponse();
}

Should be:

if (is_array($response->getResponse())) {
    $element['user_answer']['#default_value'] = $response->getResponse();
}

I”ve attached a patch with this issue.