[DomCrawler] nth-last-child incorrect behavior

``` php
$crawler = new Crawler('<html><body><ul id="test"><li>1</li><li>2</li><li>3</li></ul></body></html>');

echo "Text = " . $crawler->filter('ul#test li:nth-last-child(1)')->text();
```

This should output the number "3" (the last child), but instead outputs "2".  (nth-last-child starts at index 1).
