Fixed #17252 -- Fixed a minor regression introduced by the work in #11868,...

Fixed #17252 -- Fixed a minor regression introduced by the work in #11868, where the default sorted columns wouldn't correctly be visually represented in the changelist table headers if those columns referred to non model fields. Thanks to sebastian for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17143 bcc190cf-cafb-0310-a4f2-bffc1f526a37