NettyHeadersAdapter::putAll does not overwrite headers

NettyHeadersAdapter::putAll uses HttpHeaders::add, which does not
overwrite existing headers. putAll should use HttpHeaders::set instead,
just like NettyHeadersAdapter::put does.

Closes gh-27887

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

@@ -147,7 +147,7 @@ class NettyHeadersAdapter implements MultiValueMap<String, String> {
 
 	@Override
 	public void putAll(Map<? extends String, ? extends List<String>> map) {
-		map.forEach(this.headers::add);
+		map.forEach(this.headers::set);
 	}
 
 	@Override