Bug 18697: Fix date calculation for dayly frequencies in Serials

Bug 18697: Fix date calculation for dayly frequencies in Serials

The changes in _get_next_date_day are actually only cosmetic. The sub
now reads exactly the same as its counterparts for other units, but
the results are exactly the same as before.

In GetFictiveIssueNumber we now call _delta_units for each type of unit.
The two Delta_Days calls are moved to _delta_units. Note that this also
is a cosmetic change; results should be exactly the same.

Test plan:
[1] Edit a subscription. Test predication pattern for some daily freq.
[2] Run t/db_dependent/GetNextDate.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>