Fix Map.put contract for HeadersAdapter impl.

This commit fixes the `Map.put` contract for both Reactor Netty and
Tomcat specific `HeadersAdapter` implementations.

Issue: SPR-17415

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

@@ -124,7 +124,7 @@ class NettyHeadersAdapter implements MultiValueMap<String, String> {
 	@Override
 	public List<String> put(String key, @Nullable List<String> value) {
 		List<String> previousValues = this.headers.getAll(key);
-		this.headers.add(key, value);
+		this.headers.set(key, value);
 		return previousValues;
 	}
 

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

@@ -138,6 +138,7 @@ class TomcatHeadersAdapter implements MultiValueMap<String, String> {
 	@Nullable
 	public List<String> put(String key, List<String> value) {
 		List<String> previousValues = get(key);
+		this.headers.removeHeader(key);
 		value.forEach(v -> this.headers.addValue(key).setString(v));
 		return previousValues;
 	}