Investigate test failures in formdata.htm

[KiChjang]

The XHR responseText is empty on all responses when running the WPT test. We should investigate what's wrong here. Possible areas of failure include: 1) the Python script is not returning values properly, 2) XHR is not serializing FormData properly.

cc @Scorpil

Code: components/script/dom/formdata.rs, components/script/dom/xmlhttprequest.rs
Spec: https://xhr.spec.whatwg.org/#the-send()-method
Test: components/wpt/web-platform-tests/XMLHttpRequest/formdata.htm

[Scorpil]

@KiChjang tanks, will investigate.

[KiChjang]

Feel free to contact us at #servo on irc.mozilla.org, or leave a comment here if you have any questions.

[KiChjang]

This appears to be a missing implementation for the send function in XHR. We currently do not send any requests that contain FormData, and in order to implement that, we'll need to fix #10605 first.

[KiChjang]

cc @Manishearth

[Manishearth]

We now have proper file support too, so after the union issue it should be straightforward to make formdata serialize with files inside it.

[jdm]

Sounds like we know the solution and it's not possible to implement it yet.

[KiChjang]

This is still blocked on #9490.

[nox]

#9490 was fixed.

[KiChjang]

The test linked is no longer failing on master.