Bug 19921: Fix update child when only one adult patron category exist

Bug 19921: Fix update child when only one adult patron category exist

We are passing the Koha::Patron::Category object to the template instead
of the categorycode.

To reproduce this bug you must test in a system which has only one
patron category of the "adult" type. View the details of a patron with a
child-type patron category and choose More -> Update child to adult
patron.

This results in an error:

Can't call method "category_type" on an undefined value at
/home/vagrant/kohaclone/members/update-child.pl line 84.

The URL of the error page shows a problem with the parameters being
passed:

members/update-child.pl?op=update&borrowernumber=12345&catcode=Koha::Patron::Category=HASH(0xa168a18)&catcode_multi=

Test plan:
Make sure you have only 1 adult patron category
Update a child to adult
=> With this patch applied the error is gone and the patron has been
correctly updated

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>