PrimitiveString->import must be protected

PrimitiveString:

``` php
    class PrimitiveString extends FiltrablePrimitive
    {
        // ...

        public function import($scope)
        {
            // ...
        }
    }
```

BasePrimitive:

``` php
    abstract class BasePrimitive
    {
        // ...

        protected function import($scope)
        {
            // ...
        }
    }
```
