Mark MessageListener as ready on prefetch start

Setting MessageListener before starting prefetch would
not mark the message listener as ready. Therefore start()
method should do this work in order to get pre-fetch
going even when number of messages to pre-fetch is set to 0.

Closes #86

Author: Buddhike de Silva

src/main/java/com/amazon/sqs/javamessaging/SQSMessageConsumerPrefetch.java

@@ -552,7 +552,7 @@ void start() {
         }
         synchronized (stateLock) {
             running = true;
-            notifyStateChange();
+            messageListenerReady();
         }
     }
 

src/test/java/com/amazon/sqs/javamessaging/SQSMessageConsumerPrefetchTest.java

@@ -1930,6 +1930,22 @@ public javax.jms.Message call() throws Exception {
         consumerPrefetch.run();
     }
 
+    /**
+     * Test SetMessageListener before starting prefetch
+     * Setting MessageListener before starting prefetch would not mark
+     * the message listener as ready. Therefore start() method should
+     * do this work in order to get pre-fetch going even when
+     * number of messages to pre-fetch is set to 0.
+     */
+    @Test
+    public void testSetMessageListenerBeforeStart() {
+
+        MessageListener msgListener = mock(MessageListener.class);
+        consumerPrefetch.setMessageListener(msgListener);
+        consumerPrefetch.start();
+        assertEquals(1, consumerPrefetch.messagesRequested);
+    }
+
     /*
      * Utility functions
      */