Bug 33973: Restore sorting on Vue tables

Bug 33973: Restore sorting on Vue tables

Regression from the KohaTable Vue component added by bug 33066.

_dt_default_ajax expects options.columns to contain the columns.

Test plan:
Add some agreements, go the the agreement list view and confirm that you
can now sort the agreements using the header columns.

Note that the CSS is not correct yet. My understanding is that the
DataTable Vue component keep the 'sorting' class on the th which display
the grey up/down icon over the blue icon which indicate the current sort
column.

Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>