fix: add protocols fallback for Firefox (#20)

On the latest Firefox and Edge versions, with the current version of sockette.js, the usage of opts.protocols without any check causes the browser to send "undefined" in the Sec-WebSocket-Protocol header.

Protocol being undefined, the server is likely to send a 426: No Sec-WebSocket-Protocols requested supported (except if you specify on your server side that 'undefined' is a possible protocol value).

Passing [] to the WebSocket constructor instead of undefined nullify the need for protocol check on the server side, as browsers stop sending the header.

src/index.js

@@ -22,7 +22,7 @@ export default function (url, opts) {
 	};
 
 	self.open = function () {
-		ws = new WebSocket(url, opts.protocols);
+		ws = new WebSocket(url, opts.protocols || []);
 		for (k in $) ws[k] = $[k];
 	};