group override item missing in an assignment

This bug was identified in https://tracker.moodle.org/browse/MDLQA-17324.

When there is a group override that sets a different time limit for the assignment, the information in the assignment grading summary table for the particular group still shows the default time limit instead the one set in the override.

Note: this also happens with the group cutoff date

Steps to replicate:

1. Login as admin.
2. Enable 'Site administration > Plugins > Activity modules > Assignment settings > Enable timed assignments' setting.
3. Create a course with the following settings:
   • Group mode = Separate groups
   • Force group mode = Yes
4. Create some groups in the course.
5. Enrol several students in the course and assign them to some of the groups
6. Create an assignment with the following settings:
   • Time limit = 10 minutes
   • Cutoff date = enabled, any day is fine
7. Go to Overrides in the assignment and select 'Group overrides'
8. Add a group override for a particular group with the following settings
   • Time limit = 20 minutes
   • Cutoff date = A day further in the future
9. Go back to the assignment home page
10. Select the group that we have just override in the group selector
    • expected: The time limit from the group override should be displayed in the Grading summary table (20 mins)
    • expected: The cutoff from the group override should be displayed in the Grading summary table (Override day)
    • actual:  The default time limit is still displayed (10 mins)
    • actual:  The default cutoff date is still displayed