core (xmpp): set "to" attribute to <message> element when it's missing:

when missing, profile's full jid is used as "to" attribute. This way, "to" attribute can be retrieved later without having to check if it has been set or not.

sat/core/xmpp.py

@@ -867,6 +867,10 @@ class SatMessageProtocol(xmppim.MessageProtocol):
                 .format(xml=message_elt.toXml())))
 
         client = self.parent
+
+        if not message_elt.hasAttribute(u'to'):
+            message_elt['to'] = client.jid.full()
+
         message = {}
         subject = {}
         extra = {}