fix another assignment editing bug

fixes CNVS-8100, refs CNVS-1672

This applies to complete/incomplete, and probably letter grade and gpa
scale

Test plan:
  * make a complete/incomplete assignment with 0 points possible
    * give it a due date, but no availability date
  * mark some students complete, some students incomplete
  * add an availability date
  * the complete/incomplete for each submission status should be retained

Change-Id: I84400145a57926aa2677aef515562b29a1f32463
Reviewed-on: https://gerrit.instructure.com/36383
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>