fix assignment override root_account_id setting on create
fix assignment override root_account_id setting on create

test plan:
* new assignment overrides should have root account id

refs #LS-1356

Change-Id: Ia781c45fdf94facc5dfb12df2ee952972e044573
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/246788
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Evan Battaglia <e557ffdfc4b80aab44858030b0ae5370b86cb7b1@instructure.com>
QA-Review: Evan Battaglia <e557ffdfc4b80aab44858030b0ae5370b86cb7b1@instructure.com>
Product-Review: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms