[omemo] Make sure from attr is a string

omemo/omemoplugin.py

@@ -1041,6 +1041,8 @@ class OmemoPlugin(GajimPlugin):
                 self.publish_own_devices_list(account, new=True)
                 return False
             contact_jid = stanza.getAttr('from')
+            if isinstance(contact_jid, nbxmpp.JID):
+                contact_jid = str(contact_jid)
             if contact_jid == my_jid:
                 state.set_own_devices(devices_list)
                 state.store.sessionStore.setActiveState(devices_list, my_jid)