fix selective quiz exporting

test plan:
 - have a course with one or more quizzes
 - do a content export via the API, and give the parameter
   select[quizzes][]=123
   (where 123 is the ID of a quiz)
 - the export should contain the quiz

fixes CNVS-14771

Change-Id: Ice2824418757d9ed104d077bce13470f791b7cf3
Reviewed-on: https://gerrit.instructure.com/39397
Reviewed-by: James Williams  <jamesw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>