Add validation check for stripUnknown route response option

[johnbrett]

This might save someone some time debugging in the future. Currently we can add:

server.route({
    method: 'GET',
    path: '/',
    handler: function (request, reply) {

        return reply('ok');
    },
    config: {
        response: {
            schema: Joi.string(),
            options: {
                stripUnknown: true
            }
        }
    }
});

Which actually wouldn't strip any unknown keys without the modify key set true. With this PR if we add a route with response option stripUnknown set true without modify set to true, it will throw a validation error.

[hueniverse]

You made travis sad... Just remove the trailing spaces.

[lock]

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