update achievement gain date to submission date(not grading date)

1. It makes the leaderboard more fair.
2. Tested properly, it works.
how abt. the function name ? set_obtain_date ?

On Wed, Sep 10, 2014 at 11:20 PM, Raymoond Tang <notifications@github.com>
wrote:

> In app/models/user_course.rb:
>
> > @@ -207,6 +207,7 @@ def give_achievement(ach, should_notify=true)
> >      unless uach
> >        uach = self.user_achievements.build
> >        uach.achievement = ach
> > +      uach.update_date
>
> yes, you are right. but I still wouldn't use it.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/Coursemology/coursemology.org/pull/204/files#r17367292>
> .
>