Role tests fix

GCI students were asked to run tests before and after their changes.  One student was fixing a role-related ToDo, and wanted to run role tests, but they were dying.

They were failing when is_logged_in() returned true but there was no current.session.auth. Web2py guards against this case with a check for existence of current.session.auth. We know this case does not occur in normal use else we would get the same failure outside of role tests, so should be safe to add this guard.

Note setting auth.user without session.auth.user in the role test setup has been around for two years, so this doesn't seem to be due to a change in role tests.