Fixed #5957 -- Enforce the "required" attribute on BooleanField in newforms.

This has been the documented behaviour for ages, but it wasn't correctly
implemented. A required BooleanField must be True/checked, since False values
aren't submitted. Ideal for things like "terms of service" agreements.

Backwards incompatible (since required=True is the default for all fields).

Unclear who the original patch was from, but Tai Lee and Alex have kept it up
to date recently.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7799 bcc190cf-cafb-0310-a4f2-bffc1f526a37