change namespace to reenable proper discovery
According to XEP-0363 http://xmpp.org/extensions/xep-0363.html the namespace has changed from 'eu:siacs:conversations:http:upload' to 'urn:xmpp:http:upload'.
According to XEP-0363 http://xmpp.org/extensions/xep-0363.html the namespace has changed from 'eu:siacs:conversations:http:upload' to 'urn:xmpp:http:upload'.