`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

[Alex Crichton]

cc @elliottt