Bug 12274: handle closed invoices properly on invoice details page

Bug 12274: handle closed invoices properly on invoice details page

Trivial fix for small regression (closed invoices are displayed as
"Open" on details page, and it's not possible to reopen the closed
invoice using "Save" button) introduced by bug 10613.

Test plan:

1) Create and close some invoices
2) Note that closed invoices are erroneously displayed as "Open"
on individual invoice[s] details page
3) Apply patch
4) Check previously closed invoices; their status on details page
should now be properly displayed as "Closed on ..." (and an option
for reopening would reappear as well)
5) Ensure that "Close" / "Reopen" checkboxes followed by "Save" button
do work as expected for individual open / closed invoices respectivelly.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works as described.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>