Bug 9031: (QA follow-up) Final changes to Calendar::days_between

Bug 9031: (QA follow-up) Final changes to Calendar::days_between

The crash is caused by comparing two datetimes where one datetime is
floating and the other one was not. In that case the floating is
converted. Note too that DateTime overloads comparison operators.

This patch clones the two dates first. Puts them in floating both. And
just after that starts comparing etc.

Similar small change in hours_between.

Adding a test where the parameters are swapped for days_between.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>