Session: Implement ArrayAccess

Session: Implement ArrayAccess

Now that we dropped support for PHP 5.3.3, we can do this.

The behavior of $session['foo'] when that key doesn't already exist is a
little unexpected (it implicitly assigns null), but it's the best we can
do.

Change-Id: Ibef878867d46591a8bf542139a1719dfec3b83ab