Spring NettyHeadersAdapter may be cause Cors problem

1、version： org.springframework:spring-web:jar:5.3.18:compile org.springframework.cloud:spring-cloud-starter-gateway:jar:3.1.1:compile 2、core case： NettyRoutingFilter use the：【NettyHeadersAdapter#putAll】but【this.headers::add switch to this.headers::set】 3、eg： gateway server：set the Access-Control-Allow-Origin：http://gateway.com gateway NettyRoutingFilter：return another Access-Control-Allow-Origin：http://upstream.com NettyHeadersAdapter#putAll：rewrite the Access-Control-Allow-Origin to：http://upstream.com

Comment From: bclozel

This is by design, see #27887