Improved support for provisional responses in Session

Ignore-this: e31e11c298552b2b4a067db0fd3e5b4c

darcs-hash:20091030173945-1e3d8-8f175aa82a8e8471ff7591abc8e9df41eced3eeb
Fixed typo

Ignore-this: aabfc9f048c05d805ec5b5d5a7207678

darcs-hash:20091030153052-b22ec-f99ebbcb1a5fed9aa806f95acaaaff901f8e2805