pgsql: Fix namespace handling in xpath function

Fix namespace handling in xpath function

Previously, the xml value resulting from an xpath query would not have
namespace declarations if the namespace declarations were attached to
an ancestor element in the input xml value. That means the output value
was not correct XML. Fix that by running the result value through
xmlCopyNode(), which produces the correct namespace declarations.

Author: Ali Akbar <the(dot)apaan(at)gmail(dot)com>

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6bbf75192801c0965cec28c8d93c4ed5f2e68b20

Modified Files
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src/backend/utils/adt/xml.c | 34 ++++++++++++++++++++++++++--------
src/test/regress/expected/xml.out | 15 +++++++++++++++
src/test/regress/expected/xml_1.out | 12 ++++++++++++
src/test/regress/sql/xml.sql | 2 ++
4 files changed, 55 insertions(+), 8 deletions(-)