Libxml upgrade

Creating a generic interface between Tritium and Gokogiri (https://moovweb.atlassian.net/browse/PFST-49)
Functions with switches should have the same style... Explicitly, I am referring to have this has the returns embedded in the cases despite the named returns when `SelectXPath` on line 173 of this file sets the return values in the named returns. Choose one style and stick to it.