set any_perm to true when only one perm in get_objects_for_user

[cancan101]

If I understand what the any_perm argument in get_objects_for_user means, in this case: https://github.com/lukaszb/django-guardian/blob/dceb6beb5830746b624a7ac99d93949ff263fb98/guardian/shortcuts.py#L339 we can set any_perm=True. More generally we can do this when the len of perms is 1.

This should allow for simpler SQL below (see #148).