document.open(): remove fragment before propagating entry document's URL

[TimothyGu]

This is another part of the effort to overhaul document.open() as outlined in #3818.

Tests: web-platform-tests/wpt#10817

Fixes #2555.

[domenic]

As discussed offline, we better align with Safari and Chrome if we guard the URL and history update states behind "If entry is not document" as well. So, we should do that.

Testing the difference between that and the current PR will require writing a test to show that in such cases POSTs don't get converted to GET. I'm sorry :(

[domenic]

OK, as discussed further offline, my argument about delta from today's implementations was not sensible. All implementations will have to make small changes in this area, either with this PR's current version or with my suggested tweak to it. And those changes are of similar magnitude in both cases.

I still think it might be a bit nicer to have one if statement governing all this "weird URL-related stuff". But @TimothyGu is the expert, so I will trust his judgment that this is the best path.

[annevk]

Indeed, as with the other document.open() PRs the intention is to file bugs and align implementations once the revised standard is "done". That still seems likely to happen.