Bug 13914 : Fix UNION in SQL request for reserves statistics

Bug 13914 : Fix UNION in SQL request for reserves statistics

Test plan
=========
On Reports > reservations, select all statuses as line, pickup library as column (empty) and a range of hold date. Keep the results
Run the same reports but with statuses as column and pickup library as line
You should get the same results on another display.

* Without patch, if you've got representative data, most of the time you can observe significant differences between both results

* With the patch, results are always the same (and right if you request directly your database)

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>