Spirit Airlines shares were flying on Monday after the budget carrier revealed it had struck an 11th-hour deal to extend a debt-refinancing deadline. The deadline for the airline to refinance about $1.1 billion in debt has been pushed back from Monday to late December, Spirit said in an 8-K filing after the close of trading on Friday. The budget carrier also said in the filing that on Oct. 15, it borrowed the entire available amount of $300 million under a revolving credit facility.