fix 400 error setting hidden user feature flag in API

test plan:
 - as a user with site admin privileges, enable the "LOR External
   Tools feature in /profile/settings.

fixes CNVS-15393

Change-Id: I918bdff059b4ef3b5590aa67b0991198699196fa
Reviewed-on: https://gerrit.instructure.com/40850
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>