go.net/websocket: Use protocol parameter in websocket.Dial.

Fixes golang/go#4157

R=golang-dev, rsc, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/6571064
diff --git a/websocket/client.go b/websocket/client.go
index 1eb0e9f..e59da0b 100644
--- a/websocket/client.go
+++ b/websocket/client.go
@@ -70,6 +70,9 @@
 	if err != nil {
 		return nil, err
 	}
+	if protocol != "" {
+		config.Protocol = []string{protocol}
+	}
 	return DialConfig(config)
 }