BooleanField should also parse 'on'

[rsalmaso]

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

$(form).serialize() serialize checkbox with no value to "on" or "off"

Expected behavior

Add "on" to TRUE_VALUES and "off" to FALSE_VALUES
(I don't always have the control over html/js to add value to checkboxes).

Actual behavior

Value is ignored

[tomchristie]

I'm a little lukewarm on this, but sure, let's go ahead. Seems a common enough style.