httpserver : Add some more CORS header support

Access-Control-Request-Method -> Access-Control-Allow-Methods
Access-Control-Request-Headers -> Access-Control-Allow-Headers
Access-Control-Max-Age : Caching

Signed-off-by: Calle Wilund <calle@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>