Fetch fails when guest content set Host header.

[karthik2804]

When the guest content sets the Host header, it fails with the following error

 Error: Failed to clone headers 

This is because' Host' is a forbidden header in the wasi-http spec. I believe that this was fixed a while back where the engine would remove the host header but it seems to have regressed now.

[guybedford]

This may well have regressed in the recent headers work, but this should be a simple fix, I believe what is needed is to add a forbidden headers filter after obtaining the headers list here - https://github.com/bytecodealliance/StarlingMonkey/blob/main/builtins/web/fetch/headers.cpp#L451.

And it would also be good to add a test to avoid future regressions as well.

[karthik2804]

I am not entirely sure how the tests work in this repo but I think this is a test that already tracks it(?)

StarlingMonkey/tests/wpt-harness/expectations/fetch/api/basic/request-forbidden-headers.any.js.json

Lines 35 to 36 in 523d78d

	"Host is a forbidden request header": {
	"status": "FAIL"

[guybedford]

It may well do, although this has never passed for StarlingMonnkey so that doesn't line up the comment that this was a regression, but however we get coverage is of course fine.