has_parent buggy

``` php
var_dump($pages->find("has_parent=7438, id=8383, include=all, check_access=0")->implode(", ", "id"), $pages->get(8383)->parent("id=7438")->id);
// Output:
// string(0) "" int(7438)
```

This is somehow not the case for all pages, but only some of them won't be found with `has_parent`.
