[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        | -