[1.2.X] Fixed #14764 -- specified that a set is returned by...

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

Backport of r14787 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14788 bcc190cf-cafb-0310-a4f2-bffc1f526a37