code: public HttpHeaders add(HttpHeaders headers) {
          if (headers == null) {
              throw new NullPointerException("headers");
          }
          for (Map.Entry<String, String> e: headers) {
              add(e.getKey(), e.getValue());
          }
          return this;
      }
docs: Adds all header entries of the specified {@code headers}.
  
  @return {@code this}