Improve Illuminate\Cookie\CookieJar test coverage

This pull request increases test coverage in `CookieTest` for `Illuminate\Cookie\CookieJar`.

Before (4.1 @ 42470f7274d6dcae1b75a3bf83eecd5eb2284514):

```
\Illuminate\Cookie::CookieJar
  Methods:  80.00% ( 8/10)   Lines:  85.00% ( 17/ 20)
```

After:

```
\Illuminate\Cookie::CookieJar
  Methods: 100.00% (10/10)   Lines: 100.00% ( 20/ 20)
```