Allow null values to be cached with @CacheResult

Even though the JSR-107 spec forbids to store null values, our cache
abstraction allows that behaviour with a special handled (and this is
the default behaviour).

While this was working fine with our own set of annotations, the
JSR-107 interceptor counterpart was interpreting the spec sensu strictu.

We now allow for that special case as well.

Issue: SPR-13641

spring-context/src/test/java/org/springframework/cache/config/AbstractAnnotationTests.java

@@ -89,6 +89,21 @@ public abstract class AbstractAnnotationTests {
 		assertSame(r1, r3);
 	}
 
+	public void testCacheableNull(CacheableService<?> service) throws Exception {
+		Object o1 = new Object();
+		assertNull(cm.getCache("testCache").get(o1));
+
+		Object r1 = service.cacheNull(o1);
+		Object r2 = service.cacheNull(o1);
+		Object r3 = service.cacheNull(o1);
+
+		assertSame(r1, r2);
+		assertSame(r1, r3);
+
+		assertEquals(r3, cm.getCache("testCache").get(o1).get());
+		assertNull("Cached value should be null", r3);
+	}
+
 	public void testEvict(CacheableService<?> service) throws Exception {
 		Object o1 = new Object();
 
@@ -457,6 +472,11 @@ public abstract class AbstractAnnotationTests {
 		testCacheable(cs);
 	}
 
+	@Test
+	public void testCacheableNull() throws Exception {
+		testCacheableNull(cs);
+	}
+
 	@Test
 	public void testInvalidate() throws Exception {
 		testEvict(cs);

spring-context/src/test/java/org/springframework/cache/config/AnnotatedClassCacheableService.java

@@ -40,6 +40,11 @@ public class AnnotatedClassCacheableService implements CacheableService<Object>
 		return counter.getAndIncrement();
 	}
 
+	@Override
+	public Object cacheNull(Object arg1) {
+		return null;
+	}
+
 	@Override
 	public Object conditional(int field) {
 		return null;

spring-context/src/test/java/org/springframework/cache/config/CacheableService.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.
@@ -27,6 +27,8 @@ public interface CacheableService<T> {
 
 	T cache(Object arg1);
 
+	T cacheNull(Object arg1);
+
 	void invalidate(Object arg1);
 
 	void evictEarly(Object arg1);

spring-context/src/test/java/org/springframework/cache/config/DefaultCacheableService.java

@@ -41,6 +41,12 @@ public class DefaultCacheableService implements CacheableService<Long> {
 		return counter.getAndIncrement();
 	}
 
+	@Override
+	@Cacheable("testCache")
+	public Long cacheNull(Object arg1) {
+		return null;
+	}
+
 	@Override
 	@CacheEvict("testCache")
 	public void invalidate(Object arg1) {