Reverts workaround for case-insensitive headers.

Fixed by spring-projects/spring-framework#33666

Author: spencergibb

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/filter/headers/ForwardedHeadersFilter.java

@@ -92,8 +92,7 @@ public int getOrder() {
 	@Override
 	public HttpHeaders filter(HttpHeaders input, ServerWebExchange exchange) {
 		ServerHttpRequest request = exchange.getRequest();
-		HttpHeaders original = new HttpHeaders();
-		original.putAll(input);
+		HttpHeaders original = input;
 		HttpHeaders updated = new HttpHeaders();
 
 		// copy all headers except Forwarded