add RD bit to UDP Client

[eviltik]

fix #33
fix #29

[fishcharlie]

I can confirm this should fix #29. My patch to fix #29 has this same code, except it is right above client.send(query.toBuffer(), port, dns, err => err && reject(err));.

I don't think the position of this code should make an impact, so long as it's done before client.send.

Thanks @eviltik!!

[eviltik]

Please ignore this PR. RD bit should be enable by default, but must be MUST be togglelable.