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

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

@@ -227,7 +227,13 @@ public class LinkedCaseInsensitiveMap<V> implements Map<String, V>, Serializable
 	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);
 	}