[BrowserKit] Revert "standardized cookie paths"

"(an empty path is equivalent to /)"

An empty cookie path is only equivalent to / if the cookie was set
at the root of a domain.

This reverts symfony/symfony@128468193f5d23c8ac878f973c49ed191dd82a2a.

Conflicts:

	tests/Symfony/Tests/Component/BrowserKit/CookieTest.php