Fixed error in updateRoster when the name keyword parameter is left out.

The Roster stanza object builds item elements manually, and did not handle the
case where the name attribute is set to None, which would crash SleekXMPP.

sleekxmpp/stanza/roster.py

@@ -23,7 +23,9 @@ class Roster(ElementBase):
 			if 'subscription' in items[jid]:
 				item.attrib['subscription'] = items[jid]['subscription']
 			if 'name' in items[jid]:
-				item.attrib['name'] = items[jid]['name']
+				name = items[jid]['name']
+				if name is not None:
+					item.attrib['name'] = name
 			if 'groups' in items[jid]:
 				for group in items[jid]['groups']:
 					groupxml = ET.Element('{jabber:iq:roster}group')