fix x-cache-debug header is always sent

i noticed that i had a misconception that parameters would be used in the configuration class.
I would keep the default in the Configuration class, but making it use the resolved boolean instead of the string ``%kernel.debug%``, so that it works fine when using it in the DI extension. See DoctrineBundle for an example.

the other alternative is to call ``$container->getParameterBag()->resolveValue($config['debug'])`` if you keep the ``%kernel.debug%`` string as default