rails3: include root in assignment serializer

Change-Id: I20583fb721e8c23fcdadcb09407d5a27a88103a7
Reviewed-on: https://gerrit.instructure.com/30895
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>