Follow contract of computeIfAbsent LinkedCaseInsensitiveMap

This commit makes sure that LinkedCaseInsensitiveMap::computeIfAbsent
honors the contract of the method, and also replaces the old entry if
that mapped to null.

Closes gh-26868

Author: poutsma

spring-core/src/main/java/org/springframework/util/LinkedCaseInsensitiveMap.java

@@ -227,7 +227,13 @@ public V putIfAbsent(String key, @Nullable V value) {
 	public V computeIfAbsent(String key, Function<? super String, ? extends V> mappingFunction) {
 		String oldKey = this.caseInsensitiveKeys.putIfAbsent(convertKey(key), key);
 		if (oldKey != null) {
-			return this.targetMap.get(oldKey);
+			V oldKeyValue = this.targetMap.get(oldKey);
+			if (oldKeyValue != null) {
+				return oldKeyValue;
+			}
+			else {
+				key = oldKey;
+			}
 		}
 		return this.targetMap.computeIfAbsent(key, mappingFunction);
 	}

spring-core/src/test/java/org/springframework/util/LinkedCaseInsensitiveMapTests.java

@@ -102,6 +102,8 @@ void computeIfAbsentWithExistingValue() {
 
 		assertThat(map.put("null", null)).isNull();
 		assertThat(map.putIfAbsent("NULL", "value")).isNull();
+		assertThat(map.put("null", null)).isEqualTo("value");
+		assertThat(map.computeIfAbsent("NULL", s -> "value")).isEqualTo("value");
 		assertThat(map.get("null")).isEqualTo("value");
 	}