Fix dumping week date timepoint year

A year in an ISO 8601 week date representation may not be the same as
the year in the equivalent ISO 8601 year-month-day-of-month calendar date
representation. For example,
```
1000-W01-1
```
is the same as:
```
0999-12-30
```
This caused a problem in the custom dumping when the timepoint was in the
former format and the dump string was in the latter format (the other way
around already worked). This change correctly casts the timepoint into the
right mode before dumping the year information.

@arjclark, please review.