Libxml upgrade

Creating a generic interface between Tritium and Gokogiri (https://moovweb.atlassian.net/browse/PFST-49)
Normally in a function with multiple return values including at least one error, when you return an error you want to zero out the other value(s) you return. The "zero value" for pointers/interfaces is nil. Thus in this case and the one directly under it, you should return nil, <err>.