PROV-915 Properly support RDF mode for XML exports with namespaces.

Previously we’d always try to run each record through DOMDocument again
for “pretty printing” (indentation and such). That could cause problems
in RDF mode because you usually wouldn’t define all your used
namespaces in all your mappings for the whole graph. DOMDocument
complains about undefined namespaces though. So we simply skip that
step in RDF mode, which means our formatting might be a little off, but
at least the XML is correct and complete.