How to Stream utf8 over WebSockets in NodeJS ws?

Question

I am not sure if this is a bug or not.

According to websocket/ws documentation you can use NodeJS streams.

However, my browser only receives Binary messages when doing this, despite setting encoding to utf8:

(new WebSocket.Server({port: 8080})).on('connection', function(ws){
    var duplex = WebSocket.createWebSocketStream(ws, {encoding: 'utf8'});
    var stream = require('fs').createReadStream(pathToFile, {encoding: 'utf8'});
    stream.pipe(duplex);
});

Is this a bug, or how can I stream it as utf8?