HttpHeaders#writableHttpHeaders not effective with read-only delegate

[ilgrosso]

As described in spring-cloud/spring-cloud-gateway#3570 (comment) starting with Spring Boot 3.3.5, we are observing that HttpHeaders#writableHttpHeaders is failing to return a writable instance in case the given headers argument is an instance of Spring Security's org.springframework.security.web.server.firewall.StrictServerWebExchangeFirewall.StrictFirewallServerWebExchange.StrictFirewallHttpRequest.StrictFirewallHttpHeaders

This seems to be related to 0e257b5

[ilgrosso]

I think it would be enough to cherry-pick spring-projects/spring-framework@ef77b40#diff-39bc6ab427a4045bf51ca9822ce9386c3ba11f676d3d47e33b841dc04a7c257bR77 to the branch 6.1.x

[ilgrosso]

Closing here, moving to spring-projects/spring-framework#33795

[rwinch]

@ilgrosso Thanks for the report and correctly linking to the Spring Framework issue. For others, this is a duplicate of #15989 which is superseded by spring-projects/spring-framework#33789