Fix get_auth_method()

Fixes https://github.com/education/teachers-pet/issues/19.  Variable scope problem.

![run away](http://31.media.tumblr.com/tumblr_m5wil1Bx9u1qbaj4uo1_500.jpg)

Not sure how to test that method without crazy stubbing :confused: so will probably just hold off for https://github.com/education/teachers-pet/issues/8.