fix permissions serializer when used more than once

uncovered through rails 3 specs

Change-Id: Ic582de55532e5452a7f8bc3bbed21b551c566d12
Reviewed-on: https://gerrit.instructure.com/31786
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>