Fixed #16746 - added more HTTP code/string mappings.

This moves the arbitrary line on which HTTP codes to include away from
RFC 2616 and to the IANA assignments, thus picking up WebDAV and a couple
others.

Thanks to vfaronov for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16732 bcc190cf-cafb-0310-a4f2-bffc1f526a37