sip_im_session fix: multiple ringing responses causes ringer to continue ring after call set up

darcs-hash:20090129090903-b2873-41797e097db2503df2d299ca42491a66b43e4a9e
Spellcheck

darcs-hash:20090129100501-58252-328a8b7f8bf58ccf6797bc38fbe6d19f35b377a5