use authorization header for github api calls
use authorization header for github api calls

fixes USERS-271

test plan:
 * log in with GitHub
 * it should work

Change-Id: Ib96138aa903271c62e0ec9a50783029279d75c5c
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/225454
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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