sip_session2: fix typo

darcs-hash:20090427115456-b2873-93cc938122722d8832f944eaaac9fc0d6f379924
sip_session2: make remove_stream accept None

darcs-hash:20090427115506-b2873-d41c6dfaa123cf7e1f65bf9ce8db2b9901d8e941