Dispose on MessageSubject subscription should close socket

With the following:
```php
$client = new \Rx\Websocket\Client('ws://127.0.0.1:9090/ws');

$client->subscribe(function (MessageSubject $ms) {
    $disp = $ms->subscribe();

    addTimer(2, function () use ($disp) {
        echo "Dispose...\n";
        $disp->dispose();
    });
});
```
The websocket should close when there are no more subscriptions.