sax: make `NamespaceSupport` compliant with errata.

The original SAX 2.0.1 is not compliant with the errata of
“Namespaces in XML” for backward-compatibility reasons. Since we do not
have to support legacy software in the Nit version, implement the
errata by default.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>