Fix xpath() to return namespace definitions

Source of the bug is that libxml2's xmlNodeDump won't output XML namespace definitions that declared in the node's parents. As per https://bug639036.bugzilla-attachments.gnome.org/attachment.cgi?id=177858, the behavior is intentional.

This patch uses function xmlCopyNode that copies a node, including its namespace definitions as required (without unused namespace in the node or its children). When the copy dumped, the resulting XML is complete with its namespaces. Calling xmlCopyNode will need additional memory to execute, but reimplementing its routine to handle namespace definition will introduce much complexity to the code.