Reduce number of warnings for specs that don't reset User.current

Don't show the warning if User.current is an AnonymousUser.

Any access to User.current sets the current user to an AnonymousUser,
so without this change, the warning about User.current not being reset
is issued falsely for any test that runs code accessing User.current.
While it probably is a good idea to remove User.current everywhere,
issuing way too many warnings during specs won't accomplish this.
Merge pull request #236 from opf/feature/rails3_specs_less_user_warnings

Reduce number of warnings for specs that don't reset User.current