Fix CGBonusingObject serialization

Now state of adventure map objects that use this handler will be saved properly and they'll work as indended after save loaded. Affect following objects: Buoy, Swan Pond, Faerie Ring, etc.

lib/mapObjects/CRewardableObject.h

@@ -269,7 +269,7 @@ public:
 
 	template <typename Handler> void serialize(Handler &h, const int version)
 	{
-		h & static_cast<CGObjectInstance&>(*this);
+		h & static_cast<CRewardableObject&>(*this);
 	}
 };