Usage of layout macro with StringLoader doesn't throw any error

Following code will render into string `../@layout.latte`.

``` php
$contents = '{layout ../@layout.latte} foo';
$engine = new \Latte\Engine();
$engine->setLoader(new \Latte\Loaders\StringLoader());
echo $engine->renderToString($contents); // ../@layout.latte
```

I think there should be some error produced, doesn't it?
