Bug 25850: Adapt day of week in Koha::Calendar::get_push_amt to 0..6 with 0 being...

Bug 25850: Adapt day of week in Koha::Calendar::get_push_amt to 0..6 with 0 being Sunday.

1. Go to tools -> calendar and make sure Sundays are holidays by adding
   a holiday on a Sunday and selecting "Holiday repeated every same day
   of the week" and "Copy to all libraries" and save.
2. Go to administration -> system preferences and select "Use the
   calendar to push the due date to the next open matching weekday
   for weekly loan periods, or the next open day otherwise" for the
   system preference "useDaysMode" and save the system preferences.
3. Put the below code in a file name test.pl and execute it using
   the command "sudo koha-shell -c 'perl test.pl' kohadev"

   use Koha::Calendar;

   my $calendar = Koha::Calendar->new( branchcode => 'CPL' );
   $dt = DateTime->new(
       year       => 2020,
       month      => 06,
       day        => 21
   );

   print "This is a sunday: " .
   $dt->day_of_week . "\n";

   my $ndt = $calendar->next_open_days($dt, 0);

   print "This is a monday: " .
   $ndt->day_of_week . "\n";

4. Without the patch applied, this script
   will freeze after printing "This is a
   sunday ...".  Abort using ctrl-c.
5. Apply patch and run the script again.
   The test script will now complete.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>