fix assignment overrides double caching on update

test plan:
* due dates should not intermittently fail to update
when updating

fixes #CNVS-13067

Change-Id: Ie4faf3112c847c4e969ae59e4c782466b94a2f84
Reviewed-on: https://gerrit.instructure.com/39535
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>