Illrequest->available_backends always returned only 1 element

TEST PLAN
---------
See bug 7317 comment #20 for configuring ILL
Make sure to restart_all
Strangely, ILLIBS didn't exist, but IL did in default install,
    so use that as the patron code in your koha-conf.xml
Make sure to turn on the ILL related system preference.

1) Configure ILL.
   -- this is the hard part. Hopefully the above will give you
      enough.
2) open OPAC and log in
3) Click the 'your interlibrary loan request' in the left pane
4) Click the 'Create a new request' button.
   -- when you hover your mouse over the links for the backends,
      they will have ARRAY(...) instead of a meaningful name.
      This is what triggers the error (see comment #7).
5) Run the following commands
    git checkout -b bug_20001 origin/master
    git bz apply 20001
    restart_all
6) Refresh the OPAC page, and repeat step 4.
    -- when hovering your mouse over the links for the backends,
       they will have meaningful values now.
7) run koha qa test tools.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>