Fix WebSockets constructor with empty subprotocol

src/websocket.js

 	this.shouldAttemptReconnect = !!this.reconnectInterval;
 	this.progress = 0;
 	this.established = false;
-
-	this.socket = new WebSocket(this.url, this.options.protocols || '');
+	
+	this.socket = new WebSocket(this.url, this.options.protocols || null);
 	this.socket.binaryType = 'arraybuffer';
 	this.socket.onmessage = this.onMessage.bind(this);
 	this.socket.onopen = this.onOpen.bind(this);