fix compare_json spec helper in preparation for rails 3

Change-Id: Ibc73cddcd87012ef3776c2c5a99b305fb3c2878b
Reviewed-on: https://gerrit.instructure.com/30869
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>