wasmtime-wasi-http shouldn't add the default port to outbound host header

[lann]

The outbound HTTP implementation will add a default port to the URI authority if not present:

wasmtime/crates/wasi-http/src/http_impl.rs

Lines 68 to 69 in 1fa8de1

	} else {
	format!("{}:{port}", authority)

As reported on Zulip and as a Spin issue, this causes breakage with AWS signed requests, which include the exact host header value in the signature payload.

I believe the current behavior is incorrect. Per the HTTP/1.1 spec:

If the target URI includes an authority component, then a client MUST send a field value for Host that is identical to that authority component

[alexcrichton]

cc @elliottt