bug in dropdown()

[douglassdavis]

There appears to be a bug in dropdown(). If only $order, but not $attributes is passed in, the index 2 is incorrect for attributes and $attributes will end up having the integer value for $order. Perhaps check $order to make sure it's an array if people leave $order out of the arguments? Will fix and create pull request as soon as I can figure out how to set up my SSH keys. Let me know if I am incorrect.

   public function dropdown($title, \Closure $callback, $order = null, array $attributes = array())
    {
        $properties = compact('title', 'order', 'attributes');

        if (func_num_args() == 3) {
            $arguments = func_get_args();

            $title = Arr::get($arguments, 0);

            // If only $order is passed in without attributes, 2 is the wrong index
            $attributes = Arr::get($arguments, 2);

            $properties = compact('title', 'attributes');
        }

        $item = MenuItem::make($properties);

        call_user_func($callback, $item);

        $this->items[] = $item;

        return $item;
    }