Handle v3.1 in transport

Not sure if this is the correct solution. Here's the symptom it addresses:

	ssl := &tls.Config{
		InsecureSkipVerify: true, // FIXME: check cert
	}

	client := new(http.Client)
	client.Transport = new(spdy.Transport)
	client.Transport.(*spdy.Transport).TLSClientConfig = ssl


I start the server with ListenAndServeSPDY. When I try to connect, it crashes. 

	req, err := http.NewRequest("GET", "https://localhost:12345/konea/agent/123", nil)
	resp, err := client.Do(req)

This logs:

       (spdy debug) 2014/03/25 12:04:55 Requesting "https://localhost:12345/konea/agent/123" over SPDY.

and then crashes at transport.go:299

	stream, err := conn.Request(req, res, priority)

conn is nil here.