fix account_authorization_config fields_for for rails 3

Change-Id: I64b648d361607fe17c6d0f5befcf057b10c34cd4
Reviewed-on: https://gerrit.instructure.com/30093
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>