xpath() attempted to call xmlCopyNode() and xmlNodeDump() on a
XML_NAMESPACE_DECL, finishing with a confusing error:
=# SELECT xpath('//namespace::foo', '<root xmlns:foo="http://127.0.0.1"/>');
ERROR: 53200: could not copy node
CONTEXT: SQL function "xpath" statement 1
xpath() is changed so as it goes through xmlXPathCastNodeToString()
instead, that is able to handle namespace nodes. xml2 uses the same
solution. This issue has been discovered while digging into
9d33a5a804.
Author: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aioT7ui_ZJ9RMlfM@paquier.xyz
Backpatch-through: 14