Update MessageHeaders constructor

Following discussion with Gary.

Issue: SPR-11468

spring-messaging/src/main/java/org/springframework/messaging/MessageHeaders.java

@@ -34,6 +34,7 @@ import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
 import org.springframework.util.AlternativeJdkIdGenerator;
+import org.springframework.util.Assert;
 import org.springframework.util.IdGenerator;
 
 /**
@@ -96,24 +97,36 @@ public class MessageHeaders implements Map<String, Object>, Serializable {
 
 	private final Map<String, Object> headers;
 
-	/**
-	 * Constructs a minimal {@link MessageHeaders} with zero headers.
-	 */
-	protected MessageHeaders() {
-		this.headers = new HashMap<String, Object>();
-	}
 
 	/**
 	 * Consructs a {@link MessageHeaders} from the headers map; adding (or
 	 * overwriting) the {@link #ID} and {@link #TIMESTAMP} headers.
-	 * @param headers The map.
+	 * @param headers a map with headers to add
 	 */
 	public MessageHeaders(Map<String, Object> headers) {
+		this(headers, ((idGenerator != null) ? idGenerator : defaultIdGenerator).generateId(),
+				System.currentTimeMillis());
+	}
+
+	/**
+	 * Constructor allowing a sub-class to access the (mutable) header map as well
+	 * to provide the ID and TIMESTAMP header values.
+	 *
+	 * @param headers a map with headers to add
+	 * @param id the value for the {@link #ID} header, never {@code null}
+	 * @param timestamp the value for the {@link #TIMESTAMP} header,
+	 *    or {@code null} meaning no timestamp header
+	 */
+	protected MessageHeaders(Map<String, Object> headers, UUID id, Long timestamp) {
+		Assert.notNull(id, "'id' is required");
 		this.headers = (headers != null) ? new HashMap<String, Object>(headers) : new HashMap<String, Object>();
-		this.headers.put(ID, ((idGenerator != null) ? idGenerator : defaultIdGenerator).generateId());
-		this.headers.put(TIMESTAMP, System.currentTimeMillis());
+		this.headers.put(ID, id);
+		if (timestamp != null) {
+			this.headers.put(TIMESTAMP, timestamp);
+		}
 	}
 
+
 	protected Map<String, Object> getRawHeaders() {
 		return this.headers;
 	}

spring-messaging/src/test/java/org/springframework/messaging/MessageHeadersTests.java

@@ -142,13 +142,13 @@ public class MessageHeadersTests {
 	}
 
 	@Test
-	public void subclass() {
+	public void subClassWithCustomIdAndNoTimestamp() {
 		final AtomicLong id = new AtomicLong();
 		@SuppressWarnings("serial")
 		class MyMH extends MessageHeaders {
 
 			public MyMH() {
-				this.getRawHeaders().put(ID, new UUID(0, id.incrementAndGet()));
+				super(null, new UUID(0, id.incrementAndGet()), null);
 			}
 
 		}