Let Django handle converting expiry time in set_cookie helper (as it can handle encoding issues).

Particularly, using ``strftime`` can cause problems with some locales. The ``%b`` marker gives a localized abbreviation of the month name, which in some languages may contain non-ascii characters.
Unicode string in a header may for example result in a ``UnicodeDecodeError`` from gUnicorn when it tries to construct response.
 due to the
Merge branch 'bugfix/cookie-expiry-time-encoding' into develop