Specify versions using ~
Update Lily\Middleware\Session\*Store to ensure they should:

 - add array to session
 - overwrite array key in session
 - not overwrite other keys
 - should get array from session
Rename test files to match class names
Remove old tests for session stores
Add another test to ensure session array always added to request
Improve Lily\Middleware\Session tests
Reduce responsibility of Lily\Middleware\Session\*Store

By simplifying the responsibility of session stores and moving logic to
session middleware we can keep behaviour consistent in one place rather
than in each store.
Remove NULL values from session