ControllerNamingStrategy crashes on cb controllers

If the controller is a callback rather than a reference to some class method, the bundle crashes trying to convert a closure to a string, e.g.:

```php
$request->attributes->set('_controller', function() {
  return new Response('lol');
});
```

Additionally, i made the code look for the _attribute_ `_controller`, rather than either a get parameter, post parameter, etc called `_controller`