Improved NATDetector code in sip_audio_session

Ignore-this: fc3f91ec02c324a5eb4e6219c69d6f2e

darcs-hash:20090722140551-1e3d8-be1c5bb0d141eafc665320c8dcdebb92305b5557
Fixed typo in AudioStream._NH_RTPTransportDidFail

Ignore-this: 52758e750281e68a7962ff196c18149f

darcs-hash:20090722144025-1e3d8-330819a22465b0285dace8073b4a6dd17da52244