CONSOLE WARNING: line 11: Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/.
Test for bug 172802 - Wrong default Content-Type set in XMLHttpRequest.send(DOMString)

Should be text/plain;charset=UTF-8:

text/plain;charset=UTF-8