Only start session if hasn't already been started

On 3.0-dev I get a notice warning that a session has already been started, even though the only call to ``session_start()`` in the app is the one in the ``Session_native.php`` file.

This will only call ``session_start()`` if the session does not exist, to prevent a warning.

Signed-off-by: Dwight Watson <dwight@studentservices.com.au>