Prevent using xpath_context with a node from another document.
Prevent using xpath_context with a node from another document.

This check is contained in libxml2 too.
Throw an exception with explanatory message.

vslavik/xmlwrapp,vadz/xmlwrapp,vadz/xmlwrapp,vslavik/xmlwrapp,vslavik/xmlwrapp,vadz/xmlwrapp