clear assignment override cache after batch update

test plan:
* with rails 3 and caching enabled, create a new assignment
* add due dates for various sections
* save the assignment
* the overridden due dates should appear (i.e. the empty cache should
 not remain)

fixes #CNVS-13477

Change-Id: I1b4b2b5a9a1f91fd010b39bca6381c02bd8310fe
Reviewed-on: https://gerrit.instructure.com/37153
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>