Fix cookie path matching for empty url path

The path wouldn't match if the cookie's path was root ('/') and the
URLs path was empty.

Change-Id: I6dcd10f1fdf4f48f14e50f1b169cbdfda7005849
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>