Strictly evaluate arguments of setSession* and deleteSession.

It seems that passing \_|\_ to these functions leads to a big explosion
inside Yesod [1].  This commit makes these functions evaluate their
arguments on the spot in order to avoid these confusing errors in the future.

[1] https://github.com/prowdsponsor/mangopay/commit/aae5d92f9b3

Ping @JPMoresmau.