Call resetRequest after writeFrame for polling sessions

Previous refactoring (fcf6ae and also 43d937) in the SockJsSession
hierarchy consolidated access to the request and response in the
base class AbstractHttpSockJsSession in order to keep synchronization
concerns there. However that also unintentionally removed the call to
resetRequest() after sending a heartbeat for any of the
PollingSockJsSession classes. In general a polling session should call
resetRequest after every frame written.

This commit brings back the writeFrame override in PollingSockJsSession
with an extra call to resetRequest().

Issue: SPR-14107

Author: rstoyanchev

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/session/PollingSockJsSession.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -58,7 +58,6 @@ protected void handleRequestInternal(ServerHttpRequest request, ServerHttpRespon
 
 		if (initialRequest) {
 			writeFrame(SockJsFrame.openFrame());
-			resetRequest();
 		}
 		else if (!getMessageCache().isEmpty()) {
 			flushCache();
@@ -77,6 +76,11 @@ protected void flushCache() throws SockJsTransportFailureException {
 		SockJsMessageCodec messageCodec = getSockJsServiceConfig().getMessageCodec();
 		SockJsFrame frame = SockJsFrame.messageFrame(messageCodec, messages);
 		writeFrame(frame);
+	}
+
+	@Override
+	protected void writeFrame(SockJsFrame frame) throws SockJsTransportFailureException {
+		super.writeFrame(frame);
 		resetRequest();
 	}