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>
Bug 12274: Invoice detail: don't crash on certain billing dates

Bug 10613 sent the billingdate as a string. The template wants a DateTime
object.

To reproduce:
1/ Go on a invoice detail page
2/ Select a billing date
3/ Boom without the patch
[Tue May 20 13:39:18 2014] invoice.pl: Template process failed: undef
error - The 'day' parameter ("2014") to DateTime::new did not pass the
'an integer which is a possible valid day of month' callback.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Not all dates will make it go 'boom' but 31/07/2014 did.

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