Allow establishing of websocket with web optional protocols parameter

[jadedevin13]

cargo run --example server

1. Open web browser dev console.

2. Use ws and not secure wss, otherwise I get invalid token.

3. Run this:

   var exampleSocket = new WebSocket("ws://www.example.com/socketserver", "sip");

4. Error:

   thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: Protocol("Connection 
   reset without closing handshake")', src/libcore/result.rs:1165:5

Reference
https://developer.mozilla.org/en-US/docs/Web/API/WebSocket/WebSocket

[rofrol]

Above example does not use optional parameter

[Voronar]

@jadedevin13
I use connection callback (https://github.com/snapview/tungstenite-rs/blob/master/examples/server.rs#L12) as workaround:

req.headers().get("sec-websocket-protocol").map(|sp| {
  let headers = resp.headers_mut();
  headers.append("sec-websocket-protocol", sp.clone());
});

[daniel-abramov]

Current state: #145 (comment)