Initialize ResourceUrlProvider only once

Prior to this change, the ResourceUrlProvider would listen to
ContextRefreshedEvents and autodetect resource handlers each time. This
can cause issues when multiple contexts are involved and the last one
has no resource handler, thus clearing the previously detected ones.

This commit disables resource handlers auto-detection once some have
been detected with a refreshed context.

Issue: SPR-12592

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/ResourceUrlProvider.java

@@ -1,5 +1,5 @@
 /*
- * 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.
@@ -129,6 +129,9 @@ public class ResourceUrlProvider implements ApplicationListener<ContextRefreshed
 			if (this.handlerMap.isEmpty() && logger.isDebugEnabled()) {
 				logger.debug("No resource handling mappings found");
 			}
+			if(!this.handlerMap.isEmpty()) {
+				this.autodetect = false;
+			}
 		}
 	}