code: public static void header(Header header, String value) {
          Objects.requireNonNull(header, Required.HEADER.toString());
          
          Map<Header, String> newHeaders = new EnumMap<>(headers);
          newHeaders.put(header, value);
          headers = newHeaders;
      }
docs: Sets a custom header that is used globally on server responses
  
  @param header The name of the header
  @param value The value of the header