Fixes #25: Only add message attribute name if not already there

If someone re-uses the ReceiveMessageRequest object for multiple
calls to AmazonSQSExtendedClient, the latter will add the
RESERVED_ATTRIBUTE_NAME String as a message attribute name every
time. This can cause 413 / Request Entity Too Large errors from
AWS SQS over time.

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

@@ -344,7 +344,9 @@ public ReceiveMessageResult receiveMessage(ReceiveMessageRequest receiveMessageR
 			return super.receiveMessage(receiveMessageRequest);
 		}
 
-		receiveMessageRequest.getMessageAttributeNames().add(SQSExtendedClientConstants.RESERVED_ATTRIBUTE_NAME);
+		if (!receiveMessageRequest.getMessageAttributeNames().contains(SQSExtendedClientConstants.RESERVED_ATTRIBUTE_NAME)) {
+			receiveMessageRequest.getMessageAttributeNames().add(SQSExtendedClientConstants.RESERVED_ATTRIBUTE_NAME);
+		}
 
 		ReceiveMessageResult receiveMessageResult = super.receiveMessage(receiveMessageRequest);
 

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

@@ -25,6 +25,8 @@
 import com.amazonaws.services.sqs.AmazonSQS;
 import com.amazonaws.services.sqs.AmazonSQSClient;
 import com.amazonaws.services.sqs.model.MessageAttributeValue;
+import com.amazonaws.services.sqs.model.ReceiveMessageRequest;
+import com.amazonaws.services.sqs.model.ReceiveMessageResult;
 import com.amazonaws.services.sqs.model.SendMessageBatchRequest;
 import com.amazonaws.services.sqs.model.SendMessageBatchRequestEntry;
 import com.amazonaws.services.sqs.model.SendMessageRequest;
@@ -138,6 +140,24 @@ public void testWhenSendMessageWithSetMessageSizeThresholdThenThresholdIsHonored
         verify(mockS3, times(1)).putObject(isA(PutObjectRequest.class));
     }
 
+    @Test
+    public void testReceiveMessageMultipleTimesDoesNotAdditionallyAlterReceiveMessageRequest() {
+        ExtendedClientConfiguration extendedClientConfiguration = new ExtendedClientConfiguration()
+                .withLargePayloadSupportEnabled(mockS3, S3_BUCKET_NAME);
+        AmazonSQS sqsExtended = spy(new AmazonSQSExtendedClient(mockSqsBackend, extendedClientConfiguration));
+        when(mockSqsBackend.receiveMessage(isA(ReceiveMessageRequest.class))).thenReturn(new ReceiveMessageResult());
+
+        ReceiveMessageRequest messageRequest = new ReceiveMessageRequest();
+        ReceiveMessageRequest expectedRequest = new ReceiveMessageRequest()
+                .withMessageAttributeNames(SQSExtendedClientConstants.RESERVED_ATTRIBUTE_NAME);
+
+        sqsExtended.receiveMessage(messageRequest);
+        Assert.assertEquals(expectedRequest, messageRequest);
+
+        sqsExtended.receiveMessage(messageRequest);
+        Assert.assertEquals(expectedRequest, messageRequest);
+    }
+
     @Test
     public void testWhenMessageBatchIsSentThenOnlyMessagesLargerThanThresholdAreStoredInS3() {
         // This creates 10 messages, out of which only two are below the threshold (100K and 200K),