The ObjectTable is ignored when unshared == true

The ObjectTable and the instance cache don't mix very well in RiverMarshaller, as the objects serialized by an ObjectTable.Writer are never added to the instance cache. On the other hand, the object is looked up in the instance cache even if it does have a writer in the ObjectTable.

I've tried to work around this by using Marshaller.writeObjectUnshared(), but that doesn't work because the ObjectTable is only used when unshared == false.

I believe the ObjectTable should be used even when unshared == true, and the objects it writes should be stored in the instance cache when unshared == false, just like objects serialized by an Externalizer. In fact, SerialMarshaller seems to behave exactly like that.