[FIX] base: use context_today as default rate name

Suppose the TZ is UTC+12 and a user creates a new rate in the morning:
the date will be the day before

The default date should consider the user's timezone.

OPW-2590972

closes odoo/odoo#76513

Signed-off-by: Quentin De Paoli (qdp) <qdp@openerp.com>