[1.6.x] Fixed #21404 -- Added session.set_expiry() note to 1.6 release notes.

in Django 1.5.3, but we've changed the default to JSON in 1.6. If you upgrade

and switch from pickle to JSON, sessions created before the upgrade will be

lost. While JSON serialization does not support all Python objects like

:mod:`pickle` does, we highly recommend using JSON-serialized sessions. Also,

as JSON requires string keys, you will likely run into problems if you are

using non-string keys in ``request.session``. See the

:ref:`session_serialization` documentation for more details.

:mod:`pickle` does, we highly recommend using JSON-serialized sessions. Be

aware of the following when checking your code to determine if JSON

serialization will work for your application:

* JSON requires string keys, so you will likely run into problems if you are

  using non-string keys in ``request.session``.

* Setting session expiration by passing ``datetime`` values to

  :meth:`~django.contrib.sessions.backends.base.SessionBase.set_expiry` will

  not work as ``datetime`` values are not serializable in JSON. You can use

  integer values instead.

See the :ref:`session_serialization` documentation for more details.

Object Relational Mapper changes

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