INT-4221: Properly use Spring's Assert class

JIRA: https://jira.spring.io/browse/INT-4221

* Upgrade to those versions of Spring project dependencies which potentially will provide similar fix

**Cherry-pick to 4.3.x**

spring-integration-websocket/src/main/java/org/springframework/integration/websocket/ClientWebSocketContainer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2016 the original author or authors.
+ * Copyright 2014-2017 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.
@@ -76,7 +76,7 @@ public final class ClientWebSocketContainer extends IntegrationWebSocketContaine
 	}
 
 	public void setHeadersMap(Map<String, String> headers) {
-		Assert.notNull(headers);
+		Assert.notNull(headers, "'headers' must not be null");
 		HttpHeaders httpHeaders = new HttpHeaders();
 		for (Map.Entry<String, String> entry : headers.entrySet()) {
 			String[] values = StringUtils.commaDelimitedListToStringArray(entry.getValue());