Ignore Pong messages in StompSubProtocolHandler

Issue: SPR-12728

spring-websocket/src/main/java/org/springframework/web/socket/messaging/StompSubProtocolHandler.java

 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -28,6 +28,7 @@ import java.util.concurrent.atomic.AtomicInteger;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+
 import org.springframework.context.ApplicationEvent;
 import org.springframework.context.ApplicationEventPublisher;
 import org.springframework.context.ApplicationEventPublisherAware;
@@ -200,7 +201,7 @@ public class StompSubProtocolHandler implements SubProtocolHandler, ApplicationE
 				byteBuffer = ((BinaryMessage) webSocketMessage).getPayload();
 			}
 			else {
-				throw new IllegalArgumentException("Unexpected WebSocket message type: " + webSocketMessage);
+				return;
 			}
 
 			BufferingStompDecoder decoder = this.decoders.get(session.getId());