Bug 22874: Use false instead of 0 value for limiting on availability

Bug 22874: Use false instead of 0 value for limiting on availability

Test plan

  - Set up a elasticsearch 6 instance to work with Koha,
  - you may need to make koha works with ES 6 (see bug 20589),
  - make a search and limit it to available items only,
  => no result

  - Apply this patch,
  - make a search and limit it to available items only,
  - you should get some results

  - Do the same with ES 5.x

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>