_formatButtons(buttons) {
    return buttons && buttons.map((button) => {
      if (typeof button === 'string') {
        return {
          type: 'postback',
          title: button,
          payload: 'BOOTBOT_BUTTON_' + normalizeString(button)
        };
      } else if (button && button.type) {
        return button;
      }
      return {};
    });
  }
Used to format an array of button titles to Button objects.
@param {Array.<String|Button>} buttons If there are Button objects in the array they are not modified.