Bug 24643: Support fractional seconds

Bug 24643: Support fractional seconds

This patch allows dt_from_string to handle RFC3339 strings containing
fractional seconds up to one thousandth of a second.

Test plan:

- Apply patch
- prove t/DateUtils.t

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Joy Nelson <joy@bywatersolutions.com>
(cherry picked from commit 07aa0a562b35faaa8d04b479c33cec032ece6464)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>