implement :include_root option in as_json for rails 3

Change-Id: I4a705425461ac7a0453ac14f7627b410a1f6189b
Reviewed-on: https://gerrit.instructure.com/28773
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>