nua_session.c: changing session expiration time to follow spirit (not only letter) of RFC 4028
nua_session.c: changing session expiration time to follow spirit (not only letter) of RFC 4028

Change suggested by Timo Bruhn.

darcs-hash:20081126185840-db55f-e0924ad21e9d73fa88e6a2910bfef32bff3e5344.gz

jart/sofia-sip,unispeech/sofia-sip,unispeech/sofia-sip,xhook/sofia-sip,xhook/sofia-sip,BelledonneCommunications/sofia-sip,BelledonneCommunications/sofia-sip,unispeech/sofia-sip,erdincay/sofia-sip,xhook/sofia-sip,jart/sofia-sip,xhook/sofia-sip,BelledonneCommunications/sofia-sip,jart/sofia-sip,erdincay/sofia-sip,unispeech/sofia-sip,xhook/sofia-sip,BelledonneCommunications/sofia-sip,erdincay/sofia-sip,erdincay/sofia-sip