Changed newforms.CheckboxInput widget to return False as its value when not

include in the form (since HTML form submission doesn't send unselected check
boxes). Patch from SmileyChris. Refs #5104.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6563 bcc190cf-cafb-0310-a4f2-bffc1f526a37