only :include_root on root record in rails3 serialization

Change-Id: I1ae439ac84c364b161b5a76ec43aa55661ca3fd9
Reviewed-on: https://gerrit.instructure.com/30169
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>