_parseBody(error, body) {
    if (error) {
      return this._error(new errors.FatalError(error));
    }

    let data;
    try {
      data = JSON.parse(body.toString());
    } catch (parseError) {
      return this._error(new errors.ParseError(parseError.message));
    }

    return this.bot.processUpdate(data);
  }
Handle request body by passing it to 'callback'
@private