fix failure of quiz submissions to merge properly in user merge

fixes CNVS-12512

test plan:
  - create two users and enroll them in the same course
  - create a quiz in the course and have User_B take the quiz
  - using the user merge api:
    https://canvas.instructure.com/doc/api/users.html#method.users.merge_into
  - merge User_B into User_A, so that User_A will be the new primary user
  - check the quiz submissions and notice that the user should have submissions

Change-Id: I8c91d61b418ed486956d13f334f71684fa95cfb5
Reviewed-on: https://gerrit.instructure.com/33788
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>