Incorrect output in quiz review when Equation object is present

When viewing a fill_in_multiple_blanks quiz submission which
contains equations the answers aren't displayed in the question box.
This appears to be related to a difference in how the string is
processed when it contains equation objects.

Fixes CNVS-2669

Test Plan:
  - Create a quiz which has a fill_in_multiple_blanks quiz question and
    contains an equation object
  - Answer quiz
  - Confirm that answers are written inline, and that the field is
    readonly

Change-Id: I7c25757d3102c3c2fe38d62d615bbc71d153f756
Reviewed-on: https://gerrit.instructure.com/38986
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ryan Taylor <rtaylor@instructure.com>