protocol setter changing between special and non-special schemes

[SimonSapin]

In https://url.spec.whatwg.org/#scheme-start-state

If state override is given, run these subsubsteps:

• If url’s scheme is a special scheme and buffer is not, terminate this algorithm.
• If url’s scheme is not a special scheme and buffer is, terminate this algorithm.

I think this is a remnant from before b266a43 (before URLs with unknown/non-special schemes could be relative) and is now more strict than necessary. I suggest changing it to:

If state override is given, url’s non-relative flag is set, and buffer is a special scheme, terminate this algorithm.

In other words, the only relevant invariant left to maintain is that an URL can not be both special and non-relative.

[annevk]

I think there's another problem here specifically for Location objects. That is the leftover of #61.

[annevk]

@SimonSapin I actually think this restriction is still good or we'd have to add many more special cases. Since non-special schemes can also have an empty host and with #185 will have special parsing rules for hosts. It seems good to keep them separate.

[annevk]

My tentative plan is to close this issue through tests, if any need to be added.