According to RFC 7230, Section 5.4 states

When a proxy receives a request with an absolute-form of request-target, the proxy MUST ignore the received Host header field (if any) and instead replace it with the host information of the request-target.

That is RFC definition for what a proxy must do, but server still need the Host imformation.
eg: enabling the proxy server to distinguish among diffrent proxy policies while servicing requests for multiple host names on a single IP address.

https://tools.ietf.org/html/rfc7230#section-5.4 also says:

A client MUST send a Host header field in all HTTP/1.1 request
messages. 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

Go is forcing the two values to be the same.

In fact, there's even an old comment in the code about this in net/http/request.go:

        // RFC 2616: Must treat                                                                                                               
        //      GET /index.html HTTP/1.1                                                                                                      
        //      Host: www.google.com                                                                                                          
        // and                                                                                                                                
        //      GET http://www.google.com/index.html HTTP/1.1                                                                                 
        //      Host: doesntmatter                                                                                                            
        // the same. In the second case, any Host line is ignored.                                                                            
        req.Host = req.URL.Host
        if req.Host == "" {
                req.Host = req.Header.get("Host")
        }

So I think this is working as intended. Feel free to clarify if I misunderstood something.

There is nothing conflict with RFC.

But that has no way for me to do
curl -H "Host: domain" "http://1.1.1.1/xxx" -x "http://proxyWrittenInGo"
through a proxy written in go.

Also, as an HTTP handler, it has no way to get the raw http header Host in an HTTP request.
And I think it's inconsistent that Host field which must be in an HTTP request presents in tcpdump but disapears in httputil.DumpRequest.

Correct, Go does not permit you to get at a bogus Host header. It's bogus if it differs from an absolute request-target, as far as I understand from the spec.

If you need to test a proxy and curl doesn't let you generate the request you want to, you could probably generate it with a different program.