Remove unnecessary copying of headers in GenericMessage

Since the MessageHeaders constructor is makes a copy of the headers and
is protected against a null map, there is no need for the same to be
done in GenericMessage.

Issue: SPR-11268

spring-messaging/src/main/java/org/springframework/messaging/support/GenericMessage.java

@@ -61,12 +61,6 @@ public class GenericMessage<T> implements Message<T>, Serializable {
 	 */
 	public GenericMessage(T payload, Map<String, Object> headers) {
 		Assert.notNull(payload, "payload must not be null");
-		if (headers == null) {
-			headers = new HashMap<String, Object>();
-		}
-		else {
-			headers = new HashMap<String, Object>(headers);
-		}
 		this.headers = new MessageHeaders(headers);
 		this.payload = payload;
 	}