Fixes awslabs#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);