bug #9246 [Session] fixed wrong started state (tecbot)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #9246).

Discussion
----------

[Session] fixed wrong started state

Currently the session stays in the started mode after the call of  ```save()``` which will close the session. We found the issue in our custom SAPI which keeps the symfony stack initialized over multiple requests.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
-------

d641cd6 [Session] fixed wrong started states

src/Symfony/Component/HttpFoundation/Session/Storage/MockArraySessionStorage.php

@@ -164,6 +164,7 @@ class MockArraySessionStorage implements SessionStorageInterface
         }
         // nothing to do since we don't persist the session data
         $this->closed = false;
+        $this->started = false;
     }
 
     /**

src/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php

@@ -236,6 +236,7 @@ class NativeSessionStorage implements SessionStorageInterface
         }
 
         $this->closed = true;
+        $this->started = false;
     }
 
     /**