copy preloaded associations for overrides in rails version agnostic way

Change-Id: I690956a1cc56d94af71ff68512894e48bbe28cab
Reviewed-on: https://gerrit.instructure.com/28723
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>