sip_im_session: use default Expire when registering

darcs-hash:20090323142633-b2873-238497efd571d253b7cce1841ee1a02a279ee9b9
green.core.IncomingSessionHandler: fix AttributeError

darcs-hash:20090323142706-b2873-09a0b4561f7d68204da72987da0c893cabf7eefb