Proper Session Test

Maybe to close but especially to show a non-working example without `self::start()` in `CakeSession::delete()`. If you remove `self::start()` you will see an error occurres.

Refs: https://github.com/cakephp/cakephp/pull/4405
Unfortunately I have no controller code to provide. I use it in a very simlilar case like shown in the code changes. My use case looks like the following:

```
public function setUp() {
    CakeSession::delete('Auth');
}
public function testDeleteNoShop() {
    $this->_setAuth(ROLE_USER);
    // -> Test Code
}
public function testDeleteNoUser() {
    // -> Test Code
   // This will fail because the Auth could not be cleared by setUp()
}
```

It's simplified a lot but I think you will get an idea.