change the date calc, to set the date_due to date_due+loanlength.

change the date calc, to set the date_due to date_due+loanlength.

At the moment when you renew a document the date_due calculated is now+loanlength, but it's a bad thing, It should be
date_due+loanlength.
This patch do change Circulation.pm to use the date_due as renew base and add the loanlength to get the new date_due after
renewal.

Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>
Signed-off-by: Galen Charlton <galen.charlton@liblime.com>