Bug 17975: TT syntax for notices - Prove that HOLD_SLIP is compatible

Bug 17975: TT syntax for notices - Prove that HOLD_SLIP is compatible

Here we need to test <<today>>.
We already pass a value, but it was wrong. We must pass a string, not a
DateTime object, otherwise the KohaDates plugin will not display the
hours part if we need it.

Test plan:
Define a HOLD_SLIP notice template to match your need.
Do not forget to use
  [% today | $KohaDates %]
or
  [% today | $KohaDates with_hours => 1 %]
To access data from the reserves table, use the 'hold' variable

Tested both patches together with several date formats, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>