Thanks to aaugustin for implementing this.

(Technically this doesn't fix the original report, as we've decided against
having *any* default values, but the new feature makes it possible, and the
docs have an example addressing #15954).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16160 bcc190cf-cafb-0310-a4f2-bffc1f526a37