Fix rubric switching from empty to filled rubrics.
Fix rubric switching from empty to filled rubrics.

Fixes OUT-2552

Test Plan:
- Create an assignment with a rubric (original bug is with
  free-form comments, but should work with points too).
- Fill out the rubric for one student, but not the next student in
  speed grader.
- Open speed grader, switch to the "filled rubric" student.
- Reload the page.
- Update the rubric.
- Switch to the unfilled rubric, and back.
- Verify the filled rubric is shown.
- Repeat the process in reverse. Load the unfilled student, switch
  to filled, switch back to unfilled.

Change-Id: I77147d406ea02604705fd3b25ddd33ca546d01e5
Reviewed-on: https://gerrit.instructure.com/169081
Reviewed-by: Neil Gupta <cc4fec8a34766005dd45d48be045c08d736c11ed@instructure.com>
Reviewed-by: Augusto Callejas <7d9b9fd126d23d111f693e51511b33ea0e7192f6@instructure.com>
Tested-by: Jenkins
QA-Review: Dariusz Dzien <c9a31c8f6b0dec758954c5b3b4a6a1792cef0ed2@instructure.com>
Product-Review: Pert Eilers <b5114c9284a53681de3b74bf12eb2d826ceca963@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms