Follow contract of putIfAbsent LinkedCaseInsensitiveMap

This commit makes sure that LinkedCaseInsensitiveMap::putIfAbsent 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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -211,7 +211,13 @@ public void putAll(Map<? extends String, ? extends V> map) {
 	public V putIfAbsent(String key, @Nullable V value) {
 		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.putIfAbsent(key, value);
 	}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -99,6 +99,10 @@ void computeIfAbsentWithExistingValue() {
 		assertThat(map.computeIfAbsent("key", key2 -> "value1")).isEqualTo("value3");
 		assertThat(map.computeIfAbsent("KEY", key1 -> "value2")).isEqualTo("value3");
 		assertThat(map.computeIfAbsent("Key", key -> "value3")).isEqualTo("value3");
+
+		assertThat(map.put("null", null)).isNull();
+		assertThat(map.putIfAbsent("NULL", "value")).isNull();
+		assertThat(map.get("null")).isEqualTo("value");
 	}
 
 	@Test