INT-4244: Optimize JpaOutboundGateway for Message

JIRA: https://jira.spring.io/browse/INT-4244

The `JpaOutboundGateway` returns a whole `Message` for reply copying request headers.
Building the reply a `AbstractMessageProducingHandler` uses `MessageBuilder` to create a new `Message` with all request headers because of `shouldCopyRequestHeaders() == true`

* To avoid some overhead with new `Message` and `MessageBuilder` for nothing, just return a JPA result as is and let super class to build the Message

* Optimize all reply producers to return `MessageBuilder` to the super `handleMessage()` process
* Return the whole `gatherResult` `Message<?>` from the `ScatterGatherHandler` to have ability to transfer reply headers as well
* Mark `DelayHandler`  as `shouldCopyRequestHeaders() return false` since it doesn't modify Message and returns it as is after delay

spring-integration-core/src/main/java/org/springframework/integration/handler/DelayHandler.java

@@ -75,6 +75,7 @@ import org.springframework.util.CollectionUtils;
  * @author Mark Fisher
  * @author Artem Bilan
  * @author Gary Russell
+ *
  * @since 1.0.3
  */
 
@@ -234,6 +235,11 @@ public class DelayHandler extends AbstractReplyProducingMessageHandler implement
 		return releaseHandler;
 	}
 
+	@Override
+	protected boolean shouldCopyRequestHeaders() {
+		return false;
+	}
+
 	/**
 	 * Checks if 'requestMessage' wasn't delayed before
 	 * ({@link #releaseMessageAfterDelay} and {@link DelayHandler.DelayedMessageWrapper}).
@@ -243,10 +249,8 @@ public class DelayHandler extends AbstractReplyProducingMessageHandler implement
 	 * @param requestMessage - the Message which may be delayed.
 	 * @return - {@code null} if 'requestMessage' is delayed,
 	 *         otherwise - 'payload' from 'requestMessage'.
-	 *
 	 * @see #releaseMessage
 	 */
-
 	@Override
 	protected Object handleRequestMessage(Message<?> requestMessage) {
 		boolean delayed = requestMessage.getPayload() instanceof DelayedMessageWrapper;
@@ -260,8 +264,7 @@ public class DelayHandler extends AbstractReplyProducingMessageHandler implement
 		}
 
 		// no delay
-		Object payload = requestMessage.getPayload();
-		return delayed ? ((DelayedMessageWrapper) payload).getOriginal().getPayload() : payload;
+		return delayed ? ((DelayedMessageWrapper) requestMessage.getPayload()).getOriginal() : requestMessage;
 	}
 
 	private long determineDelayForMessage(Message<?> message) {

spring-integration-core/src/main/java/org/springframework/integration/scattergather/ScatterGatherHandler.java

@@ -145,7 +145,7 @@ public class ScatterGatherHandler extends AbstractReplyProducingMessageHandler i
 
 		Message<?> gatherResult = gatherResultChannel.receive(this.gatherTimeout);
 		if (gatherResult != null) {
-			return gatherResult.getPayload();
+			return gatherResult;
 		}
 
 		return null;

spring-integration-core/src/main/java/org/springframework/integration/transformer/ContentEnricher.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -417,7 +417,9 @@ public class ContentEnricher extends AbstractReplyProducingMessageHandler
 					targetHeaders.put(header, value);
 				}
 			}
-			return this.getMessageBuilderFactory().withPayload(targetPayload).copyHeaders(targetHeaders).build();
+			return getMessageBuilderFactory()
+					.withPayload(targetPayload)
+					.copyHeaders(targetHeaders);
 		}
 	}