Fix #16813: Restore checking whether a backend supports inctive users before...

Fix #16813: Restore checking whether a backend supports inctive users before sending inactive users in for permission checking. Thanks apollo13 for the report and poirier for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17084 bcc190cf-cafb-0310-a4f2-bffc1f526a37