Bug 24850: Correct tests for offset handling in RFC3339 dates

Bug 24850: Correct tests for offset handling in RFC3339 dates

The tests were incorrectly passing for RFC3339 dates passed with an
offset.  This patch corrects the test.

Test plan:
1/ Read the change
2/ Agree the change adheres to the RFC
3/ Run the test and varify it now fails
3/ Signoff

Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>