(one-half of tax refund attributable to jointly filed tax return is property of debtor's bankruptcy estate; the other half is attributable to non-debtor spouse; refund resulted from debtor's withholding and tax credits attributable to non-debtor spouse; court adopted presumption that spouses share in equal ownership of tax refund which can be rebutted by evidence of domestic relations court order or written, prepetition contract between spouses designating alternative ownership of tax refunds).