[BrowserKit] Fix cookie expiration on 32 bit systems
[BrowserKit] Fix cookie expiration on 32 bit systems

On 32-bit systems the cookie expiration value was not being calculated
correctly as it was being fetched as an integer. When the timestamp exceeded
the PHP_INT_MAX size it would return an invalid value, breaking the cookie
construction.

The BrowserKit cookie has now been updated to get the timestamp as a string
which works around this platform limitation.

symfony/browser-kit,symfony/BrowserKit