websocket: fix missing Sec-WebSocket-Protocol on server response.

Due to header key normalization/typo, the server never responds with
the protocol header in place. This breaks all (draft76) applications
that are using the protocol-header.

R=ukai, rsc
CC=golang-dev
https://golang.org/cl/1969046

src/pkg/websocket/server.go

@@ -133,7 +133,7 @@ func (f Handler) ServeHTTP(c *http.Conn, req *http.Request) {
 	buf.WriteString("Connection: Upgrade\r\n")
 	buf.WriteString("Sec-WebSocket-Location: " + location + "\r\n")
 	buf.WriteString("Sec-WebSocket-Origin: " + origin + "\r\n")
-	protocol, found := req.Header["Sec-WebSocket-Protocol"]
+	protocol, found := req.Header["Sec-Websocket-Protocol"]
 	if found {
 		buf.WriteString("Sec-WebSocket-Protocol: " + protocol + "\r\n")
 	}

src/pkg/websocket/websocket_test.go

@@ -130,6 +130,23 @@ func TestWithQuery(t *testing.T) {
 	ws.Close()
 }
 
+func TestWithProtocol(t *testing.T) {
+	once.Do(startServer)
+
+	client, err := net.Dial("tcp", "", serverAddr)
+	if err != nil {
+		t.Fatal("dialing", err)
+	}
+
+	ws, err := newClient("/echo", "localhost", "http://localhost",
+		"ws://localhost/echo", "test", client, handshake)
+	if err != nil {
+		t.Errorf("WebSocket handshake: %v", err)
+		return
+	}
+	ws.Close()
+}
+
 func TestHTTP(t *testing.T) {
 	once.Do(startServer)