Bug 37182: Make Koha::Statistic->new preserve 'datetime' if passed

Bug 37182: Make Koha::Statistic->new preserve 'datetime' if passed

This patch does what the title says. This behavior is tested on the
previous patch. Tests are also added for the background job (which
relies on Koha::Statistic->new and was the evidence of this
overwrite behavior).

To test:

1. Apply the regression tests patch
2. Run:
   $ ktd --shell
  k$ prove t/db_dependent/Koha/Statistic.t \
           t/db_dependent/Koha/BackgroundJob/PseudonymizeStatistic.t
=> FAIL: Tests don't pass :-/
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>