Fix PubSub namespaces

Those were broken since 9e797c1b172870b1324f2d1ff2a911c9d99a3197 as they
always used the basic PubSub namespace, i.e. without a fragment. Which
resulted in e.g. delete requests look like

<iq to="pubsub.ec-xmpp" id="2GAeW-75" type="set">
  <pubsub xmlns="http://jabber.org/protocol/pubsub">
    <delete node="2e92d38c-9e90-47f6-8e26-330d25ebe96b"/>
  </pubsub>
</iq>

when the namespace should be in fact

http://jabber.org/protocol/pubsub#owner