Bug 24528: Add a syntax to x-koha-embed to specify counts

Bug 24528: Add a syntax to x-koha-embed to specify counts

This patch adds a + syntax to specify on the x-koha-embed header that we
want the count of a relation in the response. For example:

GET /patrons/1
x-koha-embed: checkouts+count

Would return a JSON representation of a Koha::Patron, with a new
attribute added: checkouts_count, which will be the result of calling
$patron->checkouts->count.

This is all done automatically in to_api. This patch makes parsing the
x-koha-embed header build the right structure for passing to the to_api
method.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/Koha/REST/Plugin/Query.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Sponsored-by: ByWater Solutions
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>