Improved named arguments

Note to self: this is an "optimization" in the sense that it allows to generate less nodes by relying on the default values defined in the function definition.

Before:

```php
Array
(
    [0] => arg1
    [1] => Twig_Node_Expression_Constant Object(...)
    [2] => Twig_Node_Expression_Constant Object(...)
)
```

After:

```php
Array
(
    [0] => arg1
)
```