An IQ can only have a single payload

"An IQ stanza of type "get" or "set" MUST contain exactly one
 child element, which specifies the semantics of the particular
 request."

client_test.go

@@ -198,7 +198,7 @@ func bind(t *testing.T, c net.Conn, decoder *xml.Decoder) {
 	}
 
 	// TODO Check all elements
-	switch iq.Payload[0].(type) {
+	switch iq.Payload.(type) {
 	case *BindBind:
 		result := `<iq id='%s' type='result'>
   <bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'>