Fix attempts.made and attempts.remaining from being undefined & NaN on first attempt

After the first run of a Job, the internal variable Job._attempts is still undefined.  The toJSON() function includes the attempts object which references this._attempts and does math based on it.  This results in invalid information being returned (namely, attempts is undefined and remaining is NaN).

This two line change will return 0 for attempts.made if _attempts is undefined, and will return max_attempts for attempts.remaining.