Bug 17699: Use limit as end_of_month

Bug 17699: Use limit as end_of_month

From DateTime::Duration pod:
""
For positive durations, the "end_of_month" parameter defaults to wrap.
For negative durations, the default is "limit". This should match how
most people "intuitively" expect datetime math to work.
""""

We need end_of_month => limit for  positive durations as well.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>