XMLTable treats a column of a domain over xml as a non-xml column

Hi

XMLTable handles a column whose type is a domain over xml as if it were
a plain non-xml column:

== master ==

postgres=# create domain xmldom as xml;
CREATE DOMAIN
postgres=# select * from
xmltable('/r' passing '<r><a>x</a></r>'
columns c1 xml path 'a',
c2 xmldom path 'a');
c1 | c2
----------+----
<a>x</a> | x
(1 row)

Since xmldom in this examplw is a domain over xml, both columns should
return the same value,but c2 only gets the node's string value.

For boolean results xmltable already looks through domains:

postgres=# create domain intdom as int;
CREATE DOMAIN
postgres=# select * from
xmltable('/r' passing '<r/>'
columns c1 int path 'true()',
c2 intdom path 'true()',
c3 text path 'true()');
c1 | c2 | c3
----+----+------
1 | 1 | true
(1 row)

It picks 1/0 or true/false by type category,which a domain inherits, so
the intdom column behaves like the int column. The xml check should do
the same.

The cause is that XmlTableGetValue() tests typid == XMLOID, which does
not consider domains. The attached patch resolves domains to their base
type with getBaseType() before that test, and adds a regression test.

== patched ==

postgres=# select * from
xmltable('/r' passing '<r><a>x</a></r>'
columns c1 xml path 'a',
c2 xmldom path 'a');
c1 | c2
----------+----------
<a>x</a> | <a>x</a>
(1 row)

Best, Jim