Bug 15451: (followup) fix filename extension for csv file

Bug 15451: (followup) fix filename extension for csv file

This patch does the same as basket/downloadcart.pl
to set '.csv' as filename extension for downloadshelf.pl

To test:
1) Define a CSV MARC profile
2) On staff download a list, extension is '.NN'
with 'NN' the CSV profile id.
3) Apply the patch
4) Download again, check extension is now '.csv'

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