Duplicate header resolution in H1

[annevk]

Firefox rejects responses with multiple Location headers (unless their values happen to be identical). Other browsers have different handling and it likely differs per header field. It would be great to get a consistent story around this.

[mnot]

Anne, is this issue specifically about Location?

[annevk]

It basically exists for each header (e.g., I've personally tested Content-Length and Content-Type and found numerous interoperability issues).

[annevk]

See web-platform-tests/wpt#10548 and web-platform-tests/wpt#10525 for some progress on Content-Length and Content-Type by the way.

whatwg/fetch#813 notes some concerns about combining before parsing (or perhaps only about combining eliding empty/whitespace values).

[mnot]

See also #193. Can we close this in favour of that?

[reschke]

I believe so.

[annevk]

That's fine with me.