Incorrect assignment due date saved if due time is empty

[alex-shevchuk]

Try to create/edit assignments and set due time 12.00 AM. This due time has not been saved.

If user keep due time empty, but set date, to database will be saved for example: 2013-22-02 00:00:00 But need to set date to end of day - 2013-22-02 23:59:59

[artem-chechoro]

Looks good!