fix caching of non-persisted objects

refs CNVS-13848

they can't be cached, because they likely differ

Change-Id: I9f4a98f0bad4da399d02e10b0165892245584dff
Reviewed-on: https://gerrit.instructure.com/37406
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>