Instructor can't request certificate on behalf of a learner when masquerading as a learner

[graeme44]

As an instructor, masquerading as a learner who has passed the course – I am unable to request a certificate on behalf of that leaner.

The generate_user_cert function uses request.user for the student object.

There is a note in the _progress function:

# NOTE: To make sure impersonation by instructor works, use
# student instead of request.user in the rest of the function.

I believe this is how the generate_user_cert function should also work when an instructor is using the masquerade feature.