Why are request.URL.Host and Scheme blank in the development server?

I'm very new to Go. Tried this first hello, world from the documentation, and wanted to read the Host and Scheme from the request:

package hello

import (
    "fmt"
    "http"
)

func init() {
    http.HandleFunc("/", handler)
}

func handler(w http.ResponseWriter, r *http.Request) {
    fmt.Fprint(w, "Host: " + r.URL.Host + " Scheme: " + r.URL.Scheme)
}

But their values are both blank. Why?