[FIX] hr_holidays: request date tz whole company

When setting the request_date_from and request_date_to when creating a
leave that will be used to show the range of days of the leaves, we are
using the current user timezone to determine it.

eg. in UTC+2 if the leave begins at midnight on the 6th, we will see 6
and not 5 which is the day in UTC.

When creating a leave in a mode different than employee, when it is
confirmed and a leave is created for each employee, we would use the UTC
day instead which is a little unexpected.

Without the fix, the added test fails with:

    AssertionError: datetime.date(2019, 5, 5) !=
    datetime.date(2019, 5, 6) : Timezone should be kept between company
    and employee leave

opw-2573730

closes odoo/odoo#73073

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>