Fixed #15604 -- Changed django.db.models.permalink to use wraps() so that it...

Fixed #15604 -- Changed django.db.models.permalink to use wraps() so that it doesn't eat the docstring. Thanks for the report, sfllaw. Also added tests.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15798 bcc190cf-cafb-0310-a4f2-bffc1f526a37