Bug 24830: Fix parse_prefetch recursion and +count cases

Bug 24830: Fix parse_prefetch recursion and +count cases

When a child of a prefetched element wasn't found on element's
prefetch_whitelist, no prefetch element was returned.. including element
it self. Now we return element's name if no child was found.

Also, embeded keys that ended with +count where taken literaly. Now we
search the correct key by stripping _count of embedded element if it was
declared as "is_count"

To test:
1. Apply test patch but not this one
2. prove t/Koha/REST/Plugin/Query.t
CHECK => tests shoul fail
3. Apply this patch
4. prove t/Koha/REST/Plugin/Query.t
SUCCESS => tests pass
5. Sign off

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>