sip_im_session: fix for UnboundLocalError: local variable 'headers' referenced before assignment

darcs-hash:20090226131634-b2873-11258a0efcc9025a1a2123ce972d6b64e0b2b3f7
Fixed bugs in MediaTransportInitializer in session.py

darcs-hash:20090226132729-58103-ee6b11426f28fbc1e42de326264bfb0305146050