Avoid inifinite recursion in UndertowServerHttpRequest

Undertow does not provide a way to check if data is available to read
but instead we have to try to read and see if any data is returned.
This makes it impossible to implement checkOnDataAvailable without
trying to read and that can lead to infinite recursion like this:

...
UndertowServerHttpRequest$RequestBodyPublisher.checkOnDataAvailable(UndertowServerHttpRequest.java:156)
AbstractListenerReadPublisher.changeToDemandState(AbstractListenerReadPublisher.java:177)
AbstractListenerReadPublisher.access$900(AbstractListenerReadPublisher.java:47)
AbstractListenerReadPublisher$State$4.onDataAvailable(AbstractListenerReadPublisher.java:319)
AbstractListenerReadPublisher.onDataAvailable(AbstractListenerReadPublisher.java:85)
UndertowServerHttpRequest$RequestBodyPublisher.checkOnDataAvailable(UndertowServerHttpRequest.java:156)

This commit prevent the call to checkOnDataAvailable() when switching
states from READING->DEMAND which implies we exited the readAndPublish
loop because there was no more data to read.

Issue: SPR-16545

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -174,7 +174,12 @@ private boolean changeState(State oldState, State newState) {
 
 	private void changeToDemandState(State oldState) {
 		if (changeState(oldState, State.DEMAND)) {
-			checkOnDataAvailable();
+			// Protect from infinite recursion in Undertow, where we can't check if data
+			// is available, so all we can do is to try to read.
+			// Generally, no need to check if we just came out of readAndPublish()...
+			if (!oldState.equals(State.READING)) {
+				checkOnDataAvailable();
+			}
 		}
 	}