sip_im_session: fix for AttributeError regarding history_file

darcs-hash:20090128175002-b2873-f4a65828f3b149f83f70340e46818f2554a8b6af
sip_im_session fix: display_name of the account used was ignored

darcs-hash:20090128180333-b2873-48dd6e2c7b76603222867f763b42f29dd529a499