Fixes an issue where if JSONP was disabled the `Content-Type` HTTP header was still set to `application/javascript`. Props dd32, TimothyBlynJacobs. Fixes #52691. Built from https://develop.svn.wordpress.org/trunk@50695 git-svn-id: http://core.svn.wordpress.org/trunk@50304 1a063a9b-81f0-0310-95a4-ce76da25c4cd