do not show warnings when unique_id different case for same user

fixes SIS-162

TestPlan
----------
  - import user.csv with a different case than existing user' unique_id
  - no import warning errors should be shown

Change-Id: I653bcc7c2ace9953fe74ada0c0ab7da32c65662c
Reviewed-on: https://gerrit.instructure.com/30151
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Duane Johnson <duane@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Reviewed-by: Eric Adams <eadams@instructure.com>