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 <svc.cloudjenkins@instructure.com>
Reviewed-by: Evan Battaglia <ebattaglia@instructure.com>
QA-Review: Evan Battaglia <ebattaglia@instructure.com>
Product-Review: James Williams <jamesw@instructure.com>

app/models/assignment_override.rb

@@ -420,6 +420,6 @@ class AssignmentOverride < ActiveRecord::Base
   end
 
   def set_root_account_id
-    self.root_account_id ||= root_account_id
+    self.write_attribute(:root_account_id, root_account_id) unless read_attribute(:root_account_id)
   end
 end