[FIX] mrp: cast datetime from web interface

Updating a datetime field in a form view will fill the 'values' dict
with a string.
This commit casts the dates in mrp.production and mrp.workorder that are given to
their respective stock move into actual datetime object. This prevent any
converting error later in the process

Task : 2055912

closes odoo/odoo#35825

Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>