one of the 2 cookies are not cleared

Say cookie name is `foo`. When you do `ctx.session = null;`, on client side, the cookie `foo.sig` is cleared, the cookie `foo` is still there with its original value.

Why is it done this way? How to do a full clean?

Reproduce the reported case in the session middleware by setting ctx.session to null and comparing the client-side foo and foo.sig cookies. Inspect the cookie-clearing path to determine why only one cookie is removed; done means both cookies are fully cleared and the behavior is covered by a regression test.

Written by the indexing model from the issue text.