dont overwrite settings that exist on new assignments

fixes CNVS-13238

test plan:
  - on new assigment index page
  - hit the + on an assignment group to make an assignment
  - hit More Options on the modal
  - on the edit page the right assignment group should be selected
  - other assignment settings should act normally

Change-Id: Idbba3b43767aae6dd45b7d34f286cde8c1e81403
Reviewed-on: https://gerrit.instructure.com/35365
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>