The WebSocket protocol property is not always a string

[rajsite]

The protocol property of the W3CWebSocket object returns undefined in some cases while browsers will return empty string instead.

From the spec, I don't think we should ever expect the protocol property to return undefined. Ie the protocol value is empty string unless the server replies with a non-null subprotocol value and then the value is updated to a new string.

I ran into this trying to re-use some browser JS code in node.js and had to create the following shim to prevent errors:

    const W3CWebSocket = require('websocket').w3cwebsocket;
    class WebSocketShim extends W3CWebSocket {
        get protocol () {
            return this._protocol === undefined ? '' : this._protocol;
        }
    }