Improved named arguments

I've just tested again with the `array_slice` function and the behavior is not correct. As `length` has a default value, the code should work. Of course, the user is going to have a warning as `null` as a `length` is probably not allowed, but that's a PHP problem, not a Twig one. And anyway, it does not matter, as this is a PHP funciton, not a Twig one.

So, I think this change is wrong.