Transaction Sample update (#944)

1. An example that shows queue add w/ transactions should actually use
the transaction.

Author: lesv

appengine-java8/datastore/src/test/java/com/example/appengine/TransactionsTest.java

@@ -298,7 +298,7 @@ public void transactionalTaskEnqueuing() throws Exception {
     Transaction txn = datastore.beginTransaction();
     // ...
 
-    queue.add(TaskOptions.Builder.withUrl("/path/to/handler"));
+    queue.add(txn, TaskOptions.Builder.withUrl("/path/to/handler"));
 
     // ...