don't set finish date to end date unless student finished past the end date

fixes CNVS-16328

test plan:
  - as a teacher
    - create a quiz with a single question
    - give the quiz a time limit of 20 minutes
    - visit the moderate page
    - give a student a time extension of 10 minutes

  - as a student (with the extension above)
    - take the quiz and complete immediately
    - the quiz should show the correct finish time

  - make sure there are no regressions in the test plan in:
    https://gerrit.instructure.com/#/c/41236

Change-Id: Icbc65ab1710c84f1b7981710778b3e0af5e54a93
Reviewed-on: https://gerrit.instructure.com/42949
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>