Add support for RFC3311-UPDATE method

[fanshuxian]

I have a voice call, and the call will be automatically disconnected 10 minutes after it is answered.

By checking the signaling, I found that the third-party PBX（freeswitch、VOS3000 etc） will send an UPDATE request when the call lasts 600 seconds.

Sipsorcery does not support UPDATE method, and it is forced to hang up after 30 seconds because there is no media.

[ChristopheI]

Please don't hesitate to create a PR. Thx