Previously peridot would delete all cookies whenever the "Set-Cookie"
header received by get in merge-cookies was an empty list, which would
normally be the case for code like this:

    (-> (session app)
      (request "/foo")
      (request "/bar"))

Before these changes, cookies established by /foo would be missing for
/bar.

Update the tests to expire cookies instead of just dropping them on
"/delete".

Via Rob Browning <rlb@defaultvalue.org>