Fix synchronization issue in StompSubProtocolHandler

Two concurrent threads should not send a message on a single WebSocket
session at the same time, for example see:
http://docs.oracle.com/javaee/7/api/javax/websocket/RemoteEndpoint.Basic.html

In StompSubProtocolHandler it is quite possible that multiple messages
may be broadcast to a single WebSocket client concurrently.

This change adds synchronization around the sending of a message to a
specific cilent session.

Issue: SPR-11023

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

@@ -174,7 +174,11 @@ public class StompSubProtocolHandler implements SubProtocolHandler {
 		try {
 			message = MessageBuilder.withPayload(message.getPayload()).setHeaders(headers).build();
 			byte[] bytes = this.stompEncoder.encode((Message<byte[]>) message);
-			session.sendMessage(new TextMessage(new String(bytes, Charset.forName("UTF-8"))));
+
+			synchronized(session) {
+				session.sendMessage(new TextMessage(new String(bytes, Charset.forName("UTF-8"))));
+			}
+
 		}
 		catch (Throwable t) {
 			sendErrorMessage(session, t);