Bug fix in persistent set add item.

Change-Id: I0de6b27152b4384617689a1db05d9606baf79ec0

@@ -112,7 +112,7 @@ public class PersistentSet<E> implements Set<E> {
 
 
     @Override
     @Override
     public boolean add(E item) {
     public boolean add(E item) {
-        checkNotNull("Item to be added cannot be null.");
+        checkNotNull(item, "Item to be added cannot be null.");
         return items.add(serializer.encode(item));
         return items.add(serializer.encode(item));
     }
     }