Bug 17132 - Availability search broken when using Elastic

Bug 17132 - Availability search broken when using Elastic

Elastic appears to be indexing onloan as a string, but our code assumes
it is a boolean.

Test Plan:
1) Ensure you are set up using Elastic as your search engine
2) Search only for available items from the advanced search
3) Note you get no results
4) Apply this patch
5) Re-run the search
6) You should now get results!

Signed-off-by: Jennifer Schmidt <jschmidt@switchinc.org>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Mason James <mtj@kohaaloha.com>