Xmpp ping fixed to use the obtained jid.

Though a client function, ping does not use the client jid.
Fixed ping to use the obtained jid and configured server domain by default.
Client now exposes jid.

xmpp_ping.go

@@ -5,6 +5,12 @@ import (
 )
 
 func (c *Client) PingC2S(jid, server string) error {
+	if jid == "" {
+		jid = c.jid
+	}
+	if server == "" {
+		server = c.domain
+	}
 	_, err := fmt.Fprintf(c.conn, "<iq from='%s' to='%s' id='c2s1' type='get'>\n"+
 		"<ping xmlns='urn:xmpp:ping'/>\n"+
 		"</iq>",