SockJS can be instantiated with just a URL

see: https://github.com/sockjs/sockjs-client

example copied from their README:
```
var sock = new SockJS('http://mydomain.com/my_prefix');
sock.onopen = function() {
    console.log('open');
};
sock.onmessage = function(e) {
    console.log('message', e.data);
};
sock.onclose = function() {
    console.log('close');
};

sock.send('test');
sock.close();
```
Merge pull request #3206 from matthewoates/master

SockJS can be instantiated with just a URL