session2: minor change

darcs-hash:20090424112535-b2873-a8de7b3bfc97698836d89f6eddf1bc3515cae3ea
Added patch for PJSIP to deal with negative replies to re-INVITEs

darcs-hash:20090424114802-58103-7bcde8c36dfcb89cbeca22cadad15614a4dca922