code: public static SetHeaderHandler header(final HttpHandler next, final String headerName, final ExchangeAttribute headerValue) {
          return new SetHeaderHandler(next, headerName, headerValue);
      }
docs: Returns a handler that sets a response header
  
  @param next        The next handler in the chain
  @param headerName  The name of the header
  @param headerValue The header value
  @return A new set header handler