Fixed #14764 -- specified that a set is returned by User.get_..._permissions...

Fixed #14764 -- specified that a set is returned by User.get_..._permissions methods. Thanks MostAwesomeDude for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14787 bcc190cf-cafb-0310-a4f2-bffc1f526a37