[FIX] fields.date.context_today: proper signature for call by default_get()

[FIX] fields.date.context_today: proper signature for call by default_get()

The ORM automatically passes a Model instance
as the first argument to _default callables,
historically provided to give lambda functions
access to the Model instance.
context_today() does not need it but takes it
for compatibility purposes.
This also means that when called explicitly
within business code we should now pass a
Model instance as first argument, typically 'self'

bzr revid: odo@openerp.com-20120214122413-rznpdyqajrzswk4o