xhr/request-content-length.any.html is flaky on Safari and Chrome, not Firefox

[youennf]

As it is currently written,xhr/request-content-length.any.html sometimes fails as the upload to xhr/resources/echo-headers.py is sometimes (but not always) failing due to connection close.

#47809 is fixing the flakiness by adding a Content-Length header.
It would be good to get to the bottom of this issue.
I file this issue given all browsers do not behave the same here.