Added a test that captures the failure when explicit session locking is
used with file sessions.  The session tool adds a before_finalize hook
to save the session, the FileSession class asserts that the session is
locked before saving the session.  The end result is if you use
explicit locking with file sessions and release the lock yourself an
AssertionError is raised when before_finalize runs.

Fix the AssertionError that is raised when using explicit locking with
the FileSession storage class.  The before_finalize handler from the
SessionTool calls save() on the session, which asserts that the session
is locked, which may not be the case when using explicit locking.  If
explicit locking is being used FileSession's _save() will now acquire
the lock if the session is not already locked.

Fixes cherrypy#1345