Bug 29403: Fix api/v1/patrons.t

Bug 29403: Fix api/v1/patrons.t

t::lib::Dates::compare expects either correct DateTime objects, or a
system formatted datetime strings to be passed.. but as we're testing
the API, we are consistently using RFC3339 strings for date-times and
ISO8601 strings for dates.  We need to therefore pre-process the strings
into DateTime objects for the comparison.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>