Fixed #10526 -- More fixes when specifying installed apps using "foo.*".

This adds a case that was missed in r9925: underscore handling.

Backport of r10078 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10079 bcc190cf-cafb-0310-a4f2-bffc1f526a37