Mutate breaks functionality of StrictFirewallHttpHeaders with recently modified HttpHeaders#writableHttpHeaders

[MichalStehlikCz]

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.

[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"));
	}