Spring Security Mutate breaks functionality of StrictFirewallHttpHeaders with recently modified HttpHeaders#writabeHttpHeaders

Describe the bug When request wrapped by StrictServerWebExchangeFirewall / StrictFirewallHttpRequest is mutated, header sanitization provided by StrictFirewallHttpHeaders is removed

This is due to fact that - HttpHeaders#writabeHttpHeaders implementation is weird, instead of retrieval and copying of header values, it unwraps inner-most multivalue map and uses it in new (mutable) HttpHeaders instance - mutated request uses these new HttpHeaders and does not invoke http headers from delegate

Expected behavior Mutated request should be protected by StrictFirewallHttpHeaders sanitization.

Comment From: MichalStehlikCz

This test illustrates described behaviour

    @Test
    void getMutatedFirewalledExchangeGetHeaderWhenNotAllowedHeaderNameThenException() {
        this.firewall.setAllowedHeaderNames((name) -> !name.equals("bad name"));
        ServerWebExchange exchange = getFirewalledExchange();
        ServerWebExchange mutatedExchange = exchange.mutate()
                .request(exchange.getRequest().mutate().build())
                .build();
        HttpHeaders headers = mutatedExchange.getRequest().getHeaders();
        assertThatExceptionOfType(ServerExchangeRejectedException.class).isThrownBy(() -> headers.get("bad name"));
    }