Libxml upgrade

Creating a generic interface between Tritium and Gokogiri (https://moovweb.atlassian.net/browse/PFST-49)
After a quick search, it would seem this code is unnecessary. If you decide to change the the other method of access, this will need to be removed or stay as is. If we do remove it, I suggest we add a comment saying that the SetContent method is accessed directly via the embedded xml.Node object so we remember there is code for it somewhere.