don't fail module api PUT when position doesn't change

test plan:
 - enable draft state
 - go to modules page
 - publish a module and ensure the cloud icon changes
 - unpublish a module and ensure it changes back, without
   having to reload the page
 - repeat the two previous steps a couple of times

fixes CNVS-10829

Change-Id: I0806a29ccb9d1ff8e37e31e26e6eb39b3aa90fac
Reviewed-on: https://gerrit.instructure.com/29555
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>