nth-child with constant numbers not working with libxml2 2.9.1

[renato-zannon]

Hi!

The libxml2 version on ArchLinux just was bumped to 2.9.1 (from 2.8.0), and suddenly some of my tests stopped working. I've boiled down the failures a few specs.

On 2.8.0, everything passes, while on 2.9.1 the last two specs fail.

Thanks in advance!

[renato-zannon]

I didn't realize a similar bug is already being tracked on #829. Sorry for not researching properly before opening issue, I'm closing this one now.