Fixed Servlet flush

Servlet flush will now occur on the next possible write if it cannot be
done immediately.

spring-web-reactive/src/main/java/org/springframework/http/server/reactive/ServletHttpHandlerAdapter.java

@@ -247,8 +247,11 @@ public class ServletHttpHandlerAdapter extends HttpServlet {
 
 		private volatile boolean completed = false;
 
+		private volatile boolean flushOnNext = false;
+
 		private Subscription subscription;
 
+
 		public ResponseBodySubscriber(ServletAsyncContextSynchronizer synchronizer,
 				int bufferSize) {
 			this.synchronizer = synchronizer;
@@ -321,6 +324,12 @@ public class ServletHttpHandlerAdapter extends HttpServlet {
 				ServletOutputStream output = synchronizer.getResponse().getOutputStream();
 
 				boolean ready = output.isReady();
+
+				if (flushOnNext) {
+					flush(output);
+					ready = output.isReady();
+				}
+
 				logger.trace("ready: " + ready + " buffer: " + dataBuffer);
 
 				if (ready) {
@@ -370,9 +379,12 @@ public class ServletHttpHandlerAdapter extends HttpServlet {
 					logger.trace("Flushing");
 					try {
 						output.flush();
+						flushOnNext = false;
 					}
 					catch (IOException ignored) {
 					}
+				} else {
+					flushOnNext = true;
 				}
 			}