make optional fields optional for posting new assignment

fixes SIS-234

Change-Id: I37acd6c1b98a03ceaa7f84f6830ef93a11dd0072
Reviewed-on: https://gerrit.instructure.com/33630
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>