allow circular dependencies in polymorphic_type_override

test plan:
  * run specs

Change-Id: Ie6f8be9c23c76ff2ab499641c65ba5425e4a453b
Reviewed-on: https://gerrit.instructure.com/29563
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Raphael Weiner <rweiner@pivotallabs.com>
QA-Review: Raphael Weiner <rweiner@pivotallabs.com>