Update the cookie expiry whenever duration or createdAt change

The cookie is reset everytime something in the session payload
changes. Whenever its reset, the original value of expires is
used.

This is good unless the duration or the creation time change. In
that case, we need to also update the cookie expiry to match the
expiry of the signed payload.
Merge pull request #56 from fmarier/matching_expiries

Update the cookie expiry whenever duration or createdAt change