Return previous value in UndertowHeadersAdapter's remove() method

Prior to this commit, UndertowHeadersAdapter's remove() method violated
the java.util.Map contract by always returning null.

This commit fixes this by returning the previous list stored under the
specified key, and otherwise returning null if no previous value was
present.

Closes spring-projectsgh-27592

spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHeadersAdapter.java

@@ -17,6 +17,7 @@
 package org.springframework.http.server.reactive;
 
 import java.util.AbstractSet;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.LinkedHashMap;
@@ -36,6 +37,7 @@
  * {@code MultiValueMap} implementation for wrapping Undertow HTTP headers.
  *
  * @author Brian Clozel
+ * @author Sam Brannen
  * @since 5.1.1
  */
 class UndertowHeadersAdapter implements MultiValueMap<String, String> {
@@ -131,7 +133,10 @@ public List<String> put(String key, List<String> value) {
 	@Nullable
 	public List<String> remove(Object key) {
 		if (key instanceof String) {
-			this.headers.remove((String) key);
+			Collection<String> removed = this.headers.remove((String) key);
+			if (removed != null) {
+				return new ArrayList<>(removed);
+			}
 		}
 		return null;
 	}