[BrowserKit] fixed BC break done recently

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

This PR tries to avoid a BC break that was introduced recently in the management of cookies (see #7738).

see fabpot/Silex#684