When creating a user-specific override on a quiz I am not able to save the changes.

Steps to reproduce:

1. Create a quiz with a time limit
2. Add user override
3. Select any user and uncheck "Enable" to remove the time limit
4. Press save

The expected result would be to remove the time limit for this student for this quiz.  The actual result is an error message that says "You must override at least one of the quiz settings."

Alternately, if you choose ANY other setting to modify e.g. change the number of attempts allowed and then try to save, the override will save but the time limit removal will be ignored.