Cast element label to string

Fixes an issue where passing an Object that implemented __toString() was causing an "Illegal offset type in isset or empty" error in the translation helper. E.g.

    class Item {
        public function __toString() { return $this->name; }
    }

    {% for item in items %}
    {{ former_checkbox('items[' ~ item.id ~ ']').label(item) }}
    {% endfor %}