Allow specifying "Accept" type for REST server for byte array values

Currently, byte array values are sent to the client with Content-type:application/x-java-serialized-object
See https://github.com/infinispan/infinispan/blob/master/server/rest/src/main/scala/org/infinispan/rest/Server.scala#L177

It would be nice if the default value for byte arrays was application/octet-stream because serializable types are already being sent back as application/x-java-serialized-object, which covers the case when a custom object (not byte array) is put in the cache. Or allow users to specify octet-stream by "Accept" HTTP header.