Fix no header in reply-to message (see jlavallee#41)

Author: aliesbelik

src/main/com/zeroclue/jmeter/protocol/amqp/AMQPConsumer.java

@@ -365,13 +365,18 @@ private String formatHeaders(QueueingConsumer.Delivery delivery) {
             .append(": ")
             .append(delivery.getEnvelope().getRoutingKey())
             .append("\n");
-        sb.append(DELIVERY_TAG_PARAMETER).append(": ").append(delivery.getEnvelope().getDeliveryTag()).append("\n");
+        sb.append(DELIVERY_TAG_PARAMETER)
+            .append(": ")
+            .append(delivery.getEnvelope().getDeliveryTag())
+            .append("\n");
 
-        for (String key : headers.keySet()) {
-            sb.append(key)
-                .append(": ")
-                .append(headers.get(key))
-                .append("\n");
+        if (headers != null) {
+            for (String key : headers.keySet()) {
+                sb.append(key)
+                    .append(": ")
+                    .append(headers.get(key))
+                    .append("\n");
+            }
         }
 
         return sb.toString();