fix assignment overrides applicator spec for rails 3

in rails 3, even MemoryStore marshals/unmarshalls, so we can't check object_ids

Change-Id: I53ea94418cd3a578c3fca52352d8b81b95478acb
Reviewed-on: https://gerrit.instructure.com/30830
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>