Bug 19766: (bug 19058 follow-up) Fix Preview routing slip

Bug 19766: (bug 19058 follow-up) Fix Preview routing slip

This patch fixes the following errors:
The method reserve_id is not covered by tests at /home/vagrant/kohaclone/C4/Reserves.pm line 859.
The method set is not covered by tests at /home/vagrant/kohaclone/C4/Reserves.pm line 871.
Can't call method "store" on an undefined value at /home/vagrant/kohaclone/C4/Reserves.pm line 871.

This is caused by
  commit ba1f2f93ef58c8dd935add3e215facdd4a589d12
    Bug 19058: Move C4::Reserves::GetReserveId to the Koha namespace

We are calling ->reserve_id on a Koha::Holds set.

Test plan:
- Serials -> Find subscription -> "Edit routing list" in the sidebar
- Add 1+ recipients
- Save -> "Save and preview routing slip"

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>