3.26.5 | Slumber leading-underscore workaround

#1270

The problem: Fetching course enrollments through our Enrollment API Client fails for users whose username starts with _.

Slumber is just making getattr fail for resources that start with underscore: https://github.com/samgiles/slumber/blob/master/slumber/__init__.py#L29-L34

This modifies the EnrollmentApiClient to do what slumber does after raising an AttributeError, so that we'll get back a slumber.Resource object we can use to make our request.