Fixed #9053 -- Allowed for sorting of callable and ModelAdmin methods specified in list_display (added in r8352).  Previously attempting to sort on the former would raise an exception and the latter simply didn't sort.  Also added tests for this function.  Thanks rgl and jenan.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@9211 bcc190cf-cafb-0310-a4f2-bffc1f526a37