Fix entities not having valid "isValid" states. Addresses BUKKIT-2366

src/main/java/net/minecraft/server/World.java

@@ -878,6 +878,8 @@ public abstract class World implements IBlockAccess {
 
             iworldaccess.a(entity);
         }
+
+        entity.valid = true; // CraftBukkit
     }
 
     protected void b(Entity entity) {