fix(fetch): fix headers.entries/values/forEach iteration for Set-Cookie headers

[MichaelDeBoey]

See @brophdawg11's remix-run#39

It looks like the headers.entries/headers.values/headers.forEach logic we inherited when we forked isn't quite spec compliant when it comes to the Set-Cookie header. Our current implementation blindly returns this.get(headerName) in these iterators which concatenates multiple values, but Set-Cookie is special and should instead treat each value as a standalone entry (internally via looping over getAll).

Notice how in these examples, the custom header values are combined and comma-delimited, but Set-Cookie is not.

Here's this behavior in Chrome:

And in node v18:

Welcome to Node.js v18.17.0.
Type ".help" for more information.
> let headers = new Headers([['X-Custom', '1'],['Set-Cookie', 'a=1']]);
> headers.append('X-Custom', '2');
> headers.append('Set-Cookie', 'b=2');

> JSON.stringify(Array.from(headers.entries()))
'[["set-cookie","a=1"],["set-cookie","b=2"],["x-custom","1, 2"]]'

> JSON.stringify(Array.from(headers.values()))
'["a=1","b=2","1, 2"]'

> let forEachResults = [];
> headers.forEach((value, name) => forEachResults.push([name, value]))
> JSON.stringify(forEachResults)
'[["set-cookie","a=1"],["set-cookie","b=2"],["x-custom","1, 2"]]'

> let forOfResults = [];
> for ([name, value] of headers) { forOfResults.push([name, value]); }
> JSON.stringify(forOfResults)
'[["set-cookie","a=1"],["set-cookie","b=2"],["x-custom","1, 2"]]'

Also, FWIW, there's also a new headers.getSetCookie method which I think was added to help alleviate some of the confusion - but that's not available until node 19 so it didn't feel right to add that and start relying on it since we'd still be incompatible with node 18 Response instances.

Closes remix-run/remix#4354
Relates to remix-run/remix#7150

CC/ @alanshaw @Gozala

[Gozala]

Thanks for the pr