Bug 16951: Fix Item search sorting

Bug 16951: Fix Item search sorting

Caused by
  commit ac5a1bfececb5400a77f0ebad90181f5215d5a85
    Bug 16154: CGI->multi_param - Manual changes

The change was wrong, we wanted to retrieve a scalar (the string), not
an array.
We want to retrieve a string with the different column' names, not an
array of 1 element.

Test plan:
Launch an item search and play with column sort

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 2184fa6ab1a7449857df15894c8de075aac729ec)
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
(cherry picked from commit 01f495673f61eb5fced39f1f47ea2c04b5074a35)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>