code: public Response andHeader(HttpString key, String value) {
          Objects.requireNonNull(key, Required.KEY.toString());
          this.headers.put(key, value);
  
          return this;
      }
docs: Adds an additional header to the request response. If an header
  key already exists, it will we overwritten with the latest value.
  
  @param key The header constant from Headers class (e.g. Header.CONTENT_TYPE.toString())
  @param value The header value
  
  @return A response object {@link io.mangoo.routing.Response}