curl -x http://localhost:8000/ http://golang.org/

 curl http://localhost:8000/

package main

import (
    "fmt"
    "net/http"
    "net/http/httputil"
    "net/url"
    "strings"
)

func singleJoiningSlash(a, b string) string {
    aslash := strings.HasSuffix(a, "/")
    bslash := strings.HasPrefix(b, "/")
    switch {
    case aslash && bslash:
        return a + b[1:]
    case !aslash && !bslash:
        return a + "/" + b
    }
    return a + b
}

func NewSingleHostReverseProxy(target *url.URL) *httputil.ReverseProxy {
    targetQuery := target.RawQuery
    director := func(req *http.Request) {
        req.URL.Scheme = target.Scheme
        req.URL.Host = target.Host
        req.URL.Path = singleJoiningSlash(target.Path, req.URL.Path)
        fmt.Println(req.Host)
        // this fixes the bug
        req.Host = ""
        // --
        if targetQuery == "" || req.URL.RawQuery == "" {
            req.URL.RawQuery = targetQuery + req.URL.RawQuery
        } else {
            req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
        }
    }
    return &httputil.ReverseProxy{Director: director}
}

func main() {
    url, _ := url.Parse("http://golang.org/")

    go func() {
        http.ListenAndServe(":8001", NewSingleHostReverseProxy(url))
    }()
    http.ListenAndServe(":8000", httputil.NewSingleHostReverseProxy(url))

}