Fix unsafe cast of an xmlDocPtr into a xmlNodePtr.

On 64-bit systems this has proven in my project to consistently lead to bad access memory crashes when an attempt is made to dereference a struct member which xmlDocPtr and xmlNodePtr don't hold in common. Specifically the 'namespace' member is attempted to be accessed inside [GDataXMLNode nodesForXPath:namespaces:error:] which a xmlDocPtr doesn't possess.