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

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>