code: public void setAllowHeaders(String... headers) {
          allowHeadersSet.clear();
          for (String header : headers) {
              allowHeadersSet.add(header.toLowerCase());
          }
          allowHeaders = Joiner.on(",").join(headers);
      }
docs: Set the list of headers that may be sent by the browser for a CORS request.
  
  setAllowHeaders("Content-Type", "api_key", "Csrf-Token");
  
  @param headers