fix bug with anonymous access to / with future start courses

@brianhw @shnayder 

So this is one of those fixes where it was a 4 character fix but a much longer test.

One of the tests illustrates the problem, which is exercised when an anonymous user tries to access student.views.index (/) when there's a course with a future start date and MITX_FEATURES['DISABLE_START_DATES'] = False