fix gb2 set default grade functionalty

fixes CNVS-8211

Test plan:
  In a large course:
    * load gradebook2
    * try setting the default grade for an assignment (with and without
      overwriting grades)
    * do the same but only for a section

Change-Id: Ide6ba77b11df482a34fb7206a6e42dd19eda7bc0
Reviewed-on: https://gerrit.instructure.com/29121
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>