MultipleChoiceField incorrectly indicates value / selection state in HTML form output

[arkadini]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

• in a serializer, have a MultipleChoiceField with the following four choices: '1', '2', '12', '}'
• in the HTML form representation:
  • select choice 12
  • submit

(see #4119 for an exact test case)

Expected behavior

Choice 12, and only that one, is shown as selected.

Actual behavior

All four choices are shown as selected.