Pretty RDF/XML serializer misses core namespaces

[kouralex]

Whilst the following works as expected

from rdflib import Graph
g = Graph(bind_namespaces='none').parse(data='prefix ex: <http://example.org/> ex:a ex:b "test"@en .')
g.print(format="xml")

output:

<?xml version="1.0" encoding="utf-8"?>
<rdf:RDF
   xmlns:ex="http://example.org/"
   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
>
  <rdf:Description rdf:about="http://example.org/a">
    <ex:b xml:lang="en">test</ex:b>
  </rdf:Description>
</rdf:RDF>

the following one does not (due to ns1: and ns2:):

from rdflib import Graph
g = Graph(bind_namespaces='none').parse(data='prefix ex: <http://example.org/> ex:a ex:b "test"@en .')
g.print(format="pretty-xml")

output:

<?xml version="1.0" encoding="utf-8"?>
<ns1:RDF
  xmlns:ex="http://example.org/"
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
>
  <ns1:Description ns1:about="http://example.org/a">
    <ex:b ns2:lang="en">test</ex:b>
  </ns1:Description>
</ns1:RDF>

It seems like the culprit is in how the namespaces are registered in case bind_namepaces='none' is set, as removing it makes both of them work identically.

[aucampia]

Indeed seems like a bug, please consider fixing this if you have time.