Assume default format for server caches

Caches that does not specify any MediaType are assumed to have an special media type APPLICATION_UNKNOW that uses the configured marshaller in the server. Embedded caches are already assuming to have application/x-java-object encoding, and since the HR client uses protostream by default, server caches should also use it. The side effect is clients using anything else other than protostream would need to specifically configure the marshalled format in the server.

The alternative is to assume application/octet-stream for server caches