🐛(courses) fix update_courses mass command

[sampaccoud]

Purpose

Updating only one course and updating all courses did not produce the same result... the difference originates at the fact that:

course = modulestore().get_course(course_key)

and

for course in modulestore().get_courses():
    ...

don't return the same object. Go figure...

Proposal

Since the mass update is failing, I modified it to work on the same object as the specific update.