INT-4484: Fix SyndEntryPublishedDateComparator

JIRA: https://jira.spring.io/browse/INT-4484

Fix the `SyndEntryPublishedDateComparator` to move entries without a
date into the end of the sorted list

**Cherry-pick to 5.0.x and 4.3.x**

Author: artembilan

spring-integration-feed/src/main/java/org/springframework/integration/feed/inbound/FeedEntryMessageSource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -234,7 +234,7 @@ public int compare(SyndEntry entry1, SyndEntry entry2) {
 			if (date1 == null && date2 == null) {
 				return 0;
 			}
-			return (date2 == null) ? 1 : 0;
+			return date2 == null ? -1 : 1;
 		}
 
 	}