sip_session: make :call propose audio by default

- to add chat use :call x +chat
- to use chat only :call x chat

darcs-hash:20090414073848-b2873-b54c1854a48bbc28f6bf4b72006a043101259679
Updated changelog

darcs-hash:20090414074432-58252-8a69f9426ea387fccdf9e13d3eab3c9a13af5cc6