fix rails 3 change password error

test plan:
* as an account admin, edit the password for a user's login
* the current user session should not be affected

fixes #CNVS-14016

Change-Id: I537a1b1826a09451270f00d8a51d66ba02f6a956
Reviewed-on: https://gerrit.instructure.com/37381
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>