fix saving turnitin settings

fixes CNVS-15258

test plan:
- create an assignment and enable turnitin and open the advanced options
- change different combinations of settings: check/uncheck boxes, radio
  buttons, dropdowns, and fill in fields
- no matter what settings combinations you pick, when you press "Update
  Settings", and then re-open the dialog, it should be consistent
- if you close the dialog with the x, it should not keep your changes
- when you save the assignment and go back to edit it, it should keep
  your last 'updated settings'

Change-Id: I150402907b7829474f2c2a7102d2a4039438f809
Reviewed-on: https://gerrit.instructure.com/40644
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>