fix: Change format of date_override post
fix: Change format of date_override post

Credentials needs the course certificate date override data in a
slightly different format than we were passing it before. Fix!

angelapper/edx-platform,edx/edx-platform,arbrandes/edx-platform,arbrandes/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,edx/edx-platform,edx/edx-platform,arbrandes/edx-platform,edx/edx-platform,eduNEXT/edx-platform,arbrandes/edx-platform,eduNEXT/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,angelapper/edx-platform