JetBlue Airways on Friday expressed doubts over closing its $3.8 billion merger with Spirit Airlines, saying that it might not meet certain conditions required as part of the deal before the due date.Shares of Spirit tumbled 17% after the announcement, adding to their losses of about 55% this month that were sparked by a US judge’s move to block the deal.The airline said it had informed Spirit that the merger agreement might be terminated on and after Sunday due to the unmet conditions.JetBlue said it cont ...