fix downloading of student analysis CSV

test plan:
  - Have some students take a quiz
  - You should be able to download the quiz statistics CSVs as a teacher.

refs CNVS-12023

Change-Id: Iab958dd26ed15cce48347f8a076d6a311f9595aa
Reviewed-on: https://gerrit.instructure.com/32515
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>