Don't send DTMF in RFC2833 if remote party didn't advertise it on SDP

Ignore-this: c39ffb5c041ece62f0812ffee6c94125

darcs-hash:20100108130541-56b4d-afc25b6ba6deb5dda0e93c0e8454b27a9f585169
Don't access __dict__, notification.data itself is a dictionary

Ignore-this: 97a7c1ccac15d7f99b926836747ec2fe

darcs-hash:20100108152642-56b4d-cdf3f52cbfbe2e0e80c17c82ea27a1993d938dc1