Updated sip_message.py to behave more like sip_audio_session.py

darcs-hash:20090326102439-58103-63f1e2f65330784875e51afeff6c43fefaa6e913
Don't add User-Agent or Server headers if they are already present

darcs-hash:20090326104439-58103-f2ae12ce00cae980be5f79acb4d3d2837cad6f23