Intermitten failure in /XMLHttpRequest/send-entity-body-document.htm

[jdm]

This was first observed by the author of #8812 while working on that PR, but there's no reason that code should be causing this :/

  ▶ Unexpected subtest result in /XMLHttpRequest/send-entity-body-document.htm:
  │ FAIL [expected PASS] HTML document, shift-jis
  │   → assert_equals: document should be serialized and sent as text/html;charset=UTF-8 (TEST#2) expected "text/html;charset=UTF-8" but got "text/plain;charset=UTF-8"
  │ 
  │ request/<@http://web-platform.test:8000/XMLHttpRequest/send-entity-body-document.htm:18:1
  │ Test.prototype.step@http://web-platform.test:8000/resources/testharness.js:1381:20
  │ test@http://web-platform.test:8000/resources/testharness.js:495:9
  │ request@http://web-platform.test:8000/XMLHttpRequest/send-entity-body-document.htm:13:1
  │ init@http://web-platform.test:8000/XMLHttpRequest/send-entity-body-document.htm:29:42
  └ load@http://web-platform.test:8000/XMLHttpRequest/send-entity-body-document.htm:0:1

[KiChjang]

Could it be an issue with whitespaces/DOMString conversions?

[eefriedman]

Is this not just a duplicate of #8157?

[jdm]

Oh, right.