respect "leave different" option on rubric association

fixes CNVS-11195

Test plan:
  * make an assignment with 10 points possible
  * add a rubric with 20 points possible
  * choose "leave different"
  * refresh the page
  * the assignment should still have 10 points possible

Change-Id: I0e606a946ab0c67de5878eddf949f30ad09af677
Reviewed-on: https://gerrit.instructure.com/32486
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>