pgsql: Fix handling of namespace nodes in xpath() (xml)

Fix handling of namespace nodes in xpath() (xml)

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
9d33a5a804db.

Author: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/aioT7ui_ZJ9RMlfM@paquier.xyz
Backpatch-through: 14

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/6c08cbb7a7b45171f058641c553210cd3261fe6c

Modified Files
--------------
src/backend/utils/adt/xml.c | 4 +++-
src/test/regress/expected/xml.out | 6 ++++++
src/test/regress/expected/xml_1.out | 5 +++++
src/test/regress/sql/xml.sql | 1 +
4 files changed, 15 insertions(+), 1 deletion(-)