Allow separate specification of http/dial Hosts

Enable the Host: header to be customized in a Dialer.Dial call.

This is needed to implement websocket proxies.

client.go

@@ -228,6 +228,22 @@ func (d *Dialer) Dial(urlStr string, requestHeader http.Header) (*Conn, *http.Re
 		requestHeader = h
 	}
 
+	if len(requestHeader["Host"]) > 0 {
+		// This can be used to supply a Host: header which is different from
+		// the dial address.
+		u.Host = requestHeader.Get("Host")
+
+		// Drop "Host" header
+		h := http.Header{}
+		for k, v := range requestHeader {
+			if k == "Host" {
+				continue
+			}
+			h[k] = v
+		}
+		requestHeader = h
+	}
+
 	conn, resp, err := NewClient(netConn, u, requestHeader, d.ReadBufferSize, d.WriteBufferSize)
 
 	if err != nil {

client_server_test.go

@@ -288,3 +288,36 @@ func TestRespOnBadHandshake(t *testing.T) {
 		t.Errorf("resp.Body=%s, want %s", p, expectedBody)
 	}
 }
+
+// If the Host header is specified in `Dial()`, the server must receive it as
+// the `Host:` header.
+func TestHostHeader(t *testing.T) {
+	s := newServer(t)
+	defer s.Close()
+
+	specifiedHost := make(chan string, 1)
+	origHandler := s.Server.Config.Handler
+
+	// Capture the request Host header.
+	s.Server.Config.Handler = http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			specifiedHost <- r.Host
+			origHandler.ServeHTTP(w, r)
+		})
+
+	ws, resp, err := cstDialer.Dial(s.URL, http.Header{"Host": {"testhost"}})
+	if err != nil {
+		t.Fatalf("Dial: %v", err)
+	}
+	defer ws.Close()
+
+	if resp.StatusCode != http.StatusSwitchingProtocols {
+		t.Fatalf("resp.StatusCode = %v, want http.StatusSwitchingProtocols", resp.StatusCode)
+	}
+
+	if gotHost := <-specifiedHost; gotHost != "testhost" {
+		t.Fatalf("gotHost = %q, want \"testhost\"", gotHost)
+	}
+
+	sendRecv(t, ws)
+}