Improve Illuminate\Session\Store test coverage

This pull request increases test coverage in `SessionStoreTest` for `Illuminate\Session\Store`.

According to phpunit (per 4.1 at 1fa82fae0f47b3c112c5d62fa331f89177447eac):

```
\Illuminate\Session::Store
  Methods:  67.39% (31/46)   Lines:  65.49% ( 74/113)
```

This PR provides coverage for the functions not already touched by `SessionStoreTest`:

```
\Illuminate\Session::Store
  Methods: 100.00% (46/46)   Lines: 100.00% (113/113)
```