fix rce equation editor close bug

closes CNVS-4460

test-plan:
  - open the RCE somewhere
  - type in some text
  - hit the "math equation" button
  - close the equation box without inserting anything
  - type in something else
  - save page/assignment/whatever
  - verify the content typed after aborting the math editor shows up

Change-Id: Ibb3cf0704147ba23d235156619d17ada03707595
Reviewed-on: https://gerrit.instructure.com/32569
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>