copy method

Implementation

@override
Gregorian copy({int? year, int? month, int? day, int? hour, int? minute, int? second, int? millisecond}) => Gregorian(
      year ?? this.year,
      month ?? this.month,
      day ?? this.day,
      hour ?? this.hour,
      minute ?? this.minute,
      second ?? this.second,
      millisecond ?? this.millisecond,
    );