Bug 36418: Explicitly set response to json

Bug 36418: Explicitly set response to json

To test:
1. Add some checkouts and load the issues-table
2. Select 'Check in' checkbox for that item and click "Check in selected items"
3. The row becomes read and reads "Unable to check in"
4. Reload the page, the item has actually checked in

5. Try the 'Renew' checkbox and click "Renew selected items"
6. It just spins and spins.
7. Reload the page, the item has actually renewed.
8. APPLY PATCH and restart_all
9. Try the steps again and this time everything should work properly.
10. tests these URLs
http://localhost:8081/cgi-bin/koha/svc/recall
http://localhost:8080/cgi-bin/koha/svc/club/enroll
http://localhost:8081/cgi-bin/koha/svc/mana/increment : empty response

Signed-off-by: Brendan Lawlor <blawlor@clamsnet.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>