net/http: return nicer error when Client request Host is blank

Update #4271

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6931052

src/pkg/net/http/transport.go

@@ -144,6 +144,9 @@ func (t *Transport) RoundTrip(req *Request) (resp *Response, err error) {
 		}
 		return rt.RoundTrip(req)
 	}
+	if req.URL.Host == "" {
+		return nil, errors.New("http: no Host in request URL")
+	}
 	treq := &transportRequest{Request: req}
 	cm, err := t.connectMethodForRequest(treq)
 	if err != nil {

src/pkg/net/http/transport_test.go

@@ -1062,6 +1062,20 @@ func TestTransportAltProto(t *testing.T) {
 	}
 }
 
+func TestTransportNoHost(t *testing.T) {
+	tr := &Transport{}
+	_, err := tr.RoundTrip(&Request{
+		Header: make(Header),
+		URL: &url.URL{
+			Scheme: "http",
+		},
+	})
+	want := "http: no Host in request URL"
+	if got := fmt.Sprint(err); got != want {
+		t.Errorf("error = %v; want %q", err, want)
+	}
+}
+
 var proxyFromEnvTests = []struct {
 	env     string
 	wanturl string