don't overwrite true/false question answers on update

test plan:
* import the package referenced in the ticket
* edit a true/false question in one of the quizzes
* confirm that updating does not incorrectly overwrite
 the answers

fixes #CNVS-9381

Change-Id: Ic18ae414fca73c999116509fddeef6a99638f757
Reviewed-on: https://gerrit.instructure.com/29953
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>