session2: expect the other party not to include our proposed stream in the reply to re-invite

- to reproduce ./sip_session2.py 3333 a
- then, after you hear sounds :add chat

darcs-hash:20090423093612-b2873-2735ae30e85ca417cc3b2b56e906434b7db615f1
AudioStream: disallow get_local_media() be called without arguments

darcs-hash:20090423100220-58103-0a47aa50df7417ae4c1aa5a234588d960baac2b2