Reliable null value handling in ConcurrentMapCache, GuavaCache, JCacheCache

Issue: SPR-13553
(cherry picked from commit de93290)

spring-context-support/src/main/java/org/springframework/cache/jcache/JCacheCache.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 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.
@@ -126,6 +126,10 @@ public class JCacheCache implements Cache {
 
 	@SuppressWarnings("serial")
 	private static class NullHolder implements Serializable {
+
+		private Object readResolve() {
+			return NULL_HOLDER;
+		}
 	}
 
 }