Storage type HEAP default encoding should be application/x-java-object

Storage type HEAP is supposed to be a replacement for OBJECT+BINARY, storing POJOs in the data container by default but allowing the user to store serialized keys and values by configuring an encoding other than application/x-java-object.

The current StorageConfigurationManager code doesn't work like that, and the default encoding for HEAP storage is the user marshaller's media type (by default application/x-protostream).