Subscribing with an array throws fatal
If I pass an `array` as the argument for `autobahn`'s subscribe message the following fatal error occurs:

`PHP Fatal error:  Uncaught Error: Call to a member function add() on null in .../cboden/ratchet/src/Ratchet/Wamp/TopicManager.php:49`
 
Not that I want to pass an array, but if someone were to send one from the front end the loop would stop.