Define what services (and clients?) do with unknown format parameters

if accept header contains an unknown (as opposed to not supported) type parameter, the service should not choose that option. Services should not add format parameters not specified in request header. For each format parameter, we can specify whether it is allowable for the service to return something according to the format parameter without the format parameter being specified. Clients/services should not expect that they can ignore unknown format parameters when matching types in content type.