Add serialization support for btGeneric6DofSpring2Constraint

Attempt to re-order serialization data so it can be serialized in-memory (for deep copy)

src/BulletDynamics/Dynamics/btDiscreteDynamicsWorld.cpp

@@ -1456,10 +1456,10 @@ void	btDiscreteDynamicsWorld::serialize(btSerializer* serializer)
 
 	serializeDynamicsWorldInfo(serializer);
 
-	serializeRigidBodies(serializer);
-
 	serializeCollisionObjects(serializer);
 
+	serializeRigidBodies(serializer);
+
 	serializer->finishSerialization();
 }