fix complete/incomplete assignment editing bug

fixes CNVS-1501

Test plan:
  * make a complete/incomplete assignment
  * mark some students complete, some students incomplete
  * change the number of points possible on the assignment
  * the complete/incomplete status of each submission should be retained

Change-Id: Ice61d884936c908de977e1f2abdf99cbb26210ae
Reviewed-on: https://gerrit.instructure.com/36377
Reviewed-by: Liz Abinante <labinante@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>