'Assert that correct value is returned when existing refund_amount is
zero.'

def test_get_refund_amount_when_zero(self):

    campaign = MagicMock(spec=('total_budget_dollars',))
    campaign.total_budget_dollars = 200.0
    refund_amount = get_refund_amount(campaign, self.billable_amount)
    self.assertEquals(refund_amount, (campaign.total_budget_dollars - self.billable_amount))
