make optional fields optional when creating a quiz via API

Fixes SIS-235

Change-Id: If125ac733eef4dbe9b761a9ef8ee32fec60f4a22
Reviewed-on: https://gerrit.instructure.com/33631
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Duane Johnson <duane@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>