Fix custom_fields on edit
Fix custom_fields on edit

fixes PLAT-4598

Test Plan:
 - bug is fixed

Change-Id: Ie44c163226cdfe904ace2cd2469c9c818f2cf21b
Reviewed-on: https://gerrit.instructure.com/198109
Tested-by: Jenkins
Reviewed-by: Clint Furse <b31c824c483447b2438702c2da6f729b3ebb9b50@instructure.com>
Reviewed-by: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>
QA-Review: Marc Phillips <b810f855174e0bc032432310b0808df2cf714fbc@instructure.com>
Product-Review: Marc Phillips <b810f855174e0bc032432310b0808df2cf714fbc@instructure.com>

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