[FIX] point_of_sale: field date of report.pos.order should be a datetime

The date_order field of the pos.order is a datetime, not a date
As, in report.pos.order, the date field is related to date_order of the pos.order

We usually do not commit fixes altering the model fields structure in 7.0, but this one is retro-compatible, as the database structure won't change