bug 2904: fix display of URLs in UNIMARC

bug 2904: fix display of URLs in UNIMARC

This based on a patch by Nahuel ANGELINETTI; done to
merge changes with Mason's patch for 2951.  The original
patch description is:

in the previous patch the field used for link "name" was 856$z, but the unimarc doesn't specify this, it say to use the 856$2 fi
This patch make koha to use the 856$2."

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