Fix map_sql_value_to_xml_value() to treat domains like their base types.

This was already the case for domains over arrays, but not for domains
over certain built-in types such as boolean.  The special formatting
rules for those types should apply to domains over them as well.
Per discussion.

While this is a bug fix, it's also a behavioral change that seems likely
to trip up some applications.  So no back-patch.

Pavel Stehule