code: public PayloadBuilder category(final String category) {
          if (category != null) {
              aps.put("category", category);
          } else {
              aps.remove("category");
          }
          return this;
      }
docs: Sets the category of the notification for iOS8 notification
  actions.  See 13 minutes into "What's new in iOS Notifications"
  
  Passing {@code null} removes the category.
  
  @param category the name of the category supplied to the app
  when receiving the notification
  @return  this