make optional fields optional for posting new assignment
make optional fields optional for posting new assignment

fixes SIS-234

Change-Id: I37acd6c1b98a03ceaa7f84f6830ef93a11dd0072
Reviewed-on: https://gerrit.instructure.com/33630
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Duane Johnson <0a02221ea64f4c9685145b5f44cf84715b4a8bb5@instructure.com>
Product-Review: Duane Johnson <0a02221ea64f4c9685145b5f44cf84715b4a8bb5@instructure.com>
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>

sdeleon28/canvas-lms,samuels410/greatlakes-lms,Unow/canvas-lms,efrj/canvas-lms,fronteerio/canvas-lms,samuels410/greatlakes-lms,Unow/canvas-lms,kyroskoh/canvas-lms,roxolan/canvas-lms,whalejasmine/canvas-lms,djbender/canvas-lms,SwinburneOnline/canvas-lms,greyhwndz/canvas-lms,ShreniiNepal/shrenii_lmp,venturehive/canvas-lms,redconfetti/canvas-lms,Rvor/canvas-lms,roxolan/canvas-lms,fronteerio/canvas-lms,Jyaasa/canvas-lms,sfu/canvas-lms,usmschuck/canvas,djbender/canvas-lms,Jyaasa/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,jay3126/canvas-lms,snehakachroo1/LSI_replica,skmezanul/canvas-lms,wrdsb/canvas-lms,eCNAP/CANVAS,kyroskoh/canvas-lms,jay3126/canvas-lms,antoniuslin/canvas-lms,wye0220/shindig-canvas,matematikk-mooc/canvas-lms,leftplusrightllc/canvas-lms,venturehive/canvas-lms,kyroskoh/canvas-lms,snehakachroo1/LSI_replica,fronteerio/canvas-lms,eCNAP/CANVAS,dgynn/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sdeleon28/canvas-lms,instructure/canvas-lms,grahamb/canvas-lms,jay3126/canvas-lms,ShreniiNepal/shrenii_lmp,efrj/canvas-lms,skmezanul/canvas-lms,AndranikMarkosyan/lms,shidao-fm/canvas-lms,grahamb/canvas-lms,wrdsb/canvas-lms,redconfetti/canvas-lms,antoniuslin/canvas-lms,leftplusrightllc/canvas-lms,leftplusrightllc/canvas-lms,dgynn/canvas-lms,utkarshx/canvas-lms,matematikk-mooc/canvas-lms,Jyaasa/canvas-lms,redconfetti/canvas-lms,narigone/canvas,greyhwndz/canvas-lms,SwinburneOnline/canvas-lms,HotChalk/canvas-lms,venturehive/canvas-lms,utkarshx/canvas-lms,skmezanul/canvas-lms,va7map/canvas-lms,kyroskoh/canvas-lms,sdeleon28/canvas-lms,narigone/canvas,narigone/canvas,wrdsb/canvas-lms,va7map/canvas-lms,efrj/canvas-lms,antoniuslin/canvas-lms,juoni/canvas-lms,utkarshx/canvas-lms,greyhwndz/canvas-lms,instructure/canvas-lms,Jyaasa/canvas-lms,sfu/canvas-lms,snehakachroo1/LSI_replica,gbleydon/canvas-lms,sdeleon28/canvas-lms,sfu/canvas-lms,usmschuck/canvas,samuels410/greatlakes-lms,whalejasmine/canvas-lms,grahamb/canvas-lms,shidao-fm/canvas-lms,skmezanul/canvas-lms,tgroshon/canvas-lms,roxolan/canvas-lms,Rvor/canvas-lms,AndranikMarkosyan/lms,wye0220/shindig-canvas,Rvor/canvas-lms,AndranikMarkosyan/lms,eCNAP/CANVAS,juoni/canvas-lms,shidao-fm/canvas-lms,SwinburneOnline/canvas-lms,AndranikMarkosyan/lms,snehakachroo1/LSI_replica,va7map/canvas-lms,leftplusrightllc/canvas-lms,utkarshx/canvas-lms,sfu/canvas-lms,HotChalk/canvas-lms,gbleydon/canvas-lms,ShreniiNepal/shrenii_lmp,HotChalk/canvas-lms,greyhwndz/canvas-lms,venturehive/canvas-lms,shidao-fm/canvas-lms,Rvor/canvas-lms,instructure/canvas-lms,ShreniiNepal/shrenii_lmp,jay3126/canvas-lms,grahamb/canvas-lms,redconfetti/canvas-lms,matematikk-mooc/canvas-lms,matematikk-mooc/canvas-lms,HotChalk/canvas-lms,dgynn/canvas-lms,tgroshon/canvas-lms,SwinburneOnline/canvas-lms,usmschuck/canvas,va7map/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,efrj/canvas-lms,dgynn/canvas-lms,Unow/canvas-lms,instructure/canvas-lms,juoni/canvas-lms,whalejasmine/canvas-lms,roxolan/canvas-lms,djbender/canvas-lms,usmschuck/canvas,fronteerio/canvas-lms,whalejasmine/canvas-lms,narigone/canvas,wye0220/shindig-canvas,wrdsb/canvas-lms,gbleydon/canvas-lms,sfu/canvas-lms,samuels410/greatlakes-lms,tgroshon/canvas-lms,eCNAP/CANVAS,grahamb/canvas-lms,juoni/canvas-lms,tgroshon/canvas-lms,antoniuslin/canvas-lms