use @domain_root_account as the context for a bare /api/v1/users/self
use @domain_root_account as the context for a bare /api/v1/users/self

fixes CORE-1458

test plan:
 * go to /api/v1/users/self; it should work

Change-Id: I0f929ebed3fb809d554c9476946a881124e62280
Reviewed-on: https://gerrit.instructure.com/151438
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>
Product-Review: Matt Miller <40f39f5d673cab769fc15610cc6af76be547f93d@instructure.com>
Tested-by: Jenkins

djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms