Implement sending a Document via XMLHttpRequest

[jdm]

Specification: https://xhr.spec.whatwg.org/#interface-xmlhttprequest and https://xhr.spec.whatwg.org/#dom-xmlhttprequest-send
Code: components/script/dom/webidls/XMLHttpRequest.webidl, components/script/dom/xmlhttprequest.rs (see Send method)
Tests: ./mach test-wpt tests/wpt/web-platform-tests/XMLHttpRequest/
Minimal testcase:

<script>
var x = new XMLHttpRequest();
x.open('POST', location.href, false);
x.send(document);
</script>

And run that from an http server like this minimal python one, where you can inspect the data received in the terminal window.

For implementing the serialization of the document, we should be able to use the existing HTML serialization code. See serialize in components/script/dom/element.rs for an example!

[jdm]

Note: this is reserved for a potential RGSoC applicant, so please talk with me before starting to work on this!

[jdm]

Closing in favour of #9490.