onlyDay method Null safety

1. baguette
2. DateTimeExtension
3. DateTimeExtension
4. onlyDay method

DateTimeExtension extension

1. Methods
2. copyWith
3. firstDay
4. isSameDay
5. isSameHour
6. isSameMinute
7. isSameMonth
8. isSameYear
9. lastDay
10. lastMonth
11. nextMonth
12. onlyDay
13. onlyMonth
14. onlyYear