fix assigning zero scores
fix assigning zero scores

closes QO-463

Test plan:
Create/take quiz
In Speedgrader
- attempt to give individual questions
  scores of 0
- reload page
- verify that 0 score is saved
Repeat in quiz moderate page

Change-Id: I06833fbf0955e3c71aaa74344b57e82900505798
Reviewed-on: https://gerrit.instructure.com/176960
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Reviewed-by: Brent Burgoyne <bccf16d7abdd2611580b1cd7d7a6fcc150d619da@instructure.com>
Tested-by: Jenkins
QA-Review: Gentry Beckmann <f487939f789774cca922ca5b886632de73477ea0@instructure.com>
Product-Review: Michael Brewer-Davis <785693f4c81267e5b88c0f98d39b56a39a2b6910@instructure.com>

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