What to do when URLUtils.protocol setter change the scheme "type"

[SimonSapin]

E.g. from a relative scheme to a non-relative scheme. See discussion in #83.

[SimonSapin]

See also whatwg/url#104

[nox]

web-platform-tests/wpt#4413 landed and I don't know if we have those tests in our tree.

[djc]

We do have those tests in our tree now, so presumably this can be closed now?