[FIX] project_timesheet_holidays: refusing leaves delete timesheet

When refusing leave, we can not remove linked timesheet,
since they are attached to the leave.
We need to first detach it, then remove the timesheet
entries.

Also, add a test about the removal of linked timesheets.

addons/project_timesheet_holidays/models/hr_holidays.py

@@ -61,5 +61,6 @@ class Holidays(models.Model):
     def action_refuse(self):
         """ Remove the timesheets linked to the refused holidays """
         result = super(Holidays, self).action_refuse()
+        self.mapped('timesheet_ids').write({'holiday_id': False})
         self.mapped('timesheet_ids').unlink()
         return result

addons/project_timesheet_holidays/tests/test_timesheet_holidays.py

@@ -72,6 +72,10 @@ class TestTimesheetHolidays(TestTimesheet):
         holiday.sudo().action_validate()
         self.assertEquals(len(holiday.timesheet_ids), number_of_days, 'Number of generated timesheets should be the same as the leave duration (1 per day)')
 
+        # manager refuse the leave
+        holiday.sudo().action_refuse()
+        self.assertEquals(len(holiday.timesheet_ids), 0, 'Number of linked timesheets should be zero, since the leave is refused.')
+
     def test_validate_without_timesheet(self):
         # employee creates a leave request
         number_of_days = (self.leave_end_datetime - self.leave_start_datetime).days