[calendar] very_short_weekdays fallback to datatype-date-format langage data

Fix #1796.

Both `shortWeekDays` and `weekDays` will be localized at least `datatype-date-format` is available in specified language by this change.

For example, this issue occurs when language data is not found in `calendar-base` and found in `datatype-date-format` like language `ko`.

Before (v3.17.2):
![2014-06-22 at 22 00](https://cloud.githubusercontent.com/assets/34588/3351522/a9c3e52c-fa0f-11e3-85f8-009b864e180e.png)
After (this PR):
![2014-06-22 at 22 01](https://cloud.githubusercontent.com/assets/34588/3351523/aff04ae4-fa0f-11e3-84d6-a64db298eefe.png)

Note: I'll update `HISTORY.md` after merging this PR.

/cc @brunobasto