Allows establishing of websocket with web optional protocols parameter

[jadedevin13]

The browser won't autoclose the connection since it will now see the header it requires.

#107

[agalakhov]

Sorry, the code you just pushed is not compileable.

[daniel-abramov]

Why do we need protocol variable set to an empty header value if get() returns an optional value?

It may be rewritten much simpler as:

    let mut builder = Response::builder()
        .status(StatusCode::SWITCHING_PROTOCOLS)
        .version(request.version())
        .header("Connection", "Upgrade")
        .header("Upgrade", "websocket")
        .header("Sec-WebSocket-Accept", convert_key(key.as_bytes())?);

    const PROTOCOL: &'static str = "Sec-WebSocket-Protocol";
    if let Some(protocol) = request.headers().get(PROTOCOL) {
        builder = builder.header(PROTOCOL, protocol);
    }

or something like this.

[daniel-abramov]

There seems to be no activity in this PR, so I'm going to close it. We could re-introduce the changes if we decide to use this approach. Please check #145 (comment) for further details.