chenhaodong / mew-iview

Fixes #3232

Date utility functions `prevMonth` and `nextMonth` change the source
date when using `src.set[Date|Month|FullYear]()`. This change creates a
copy of the date at entrance of function so we have a pure function.

Made some code refactor and added specs for these functions.

update TimePicker

update TimePicker

update TimePicker

update DatePicker