Spirit Airlines shares were soaring on Monday after the budget carrier revealed it had struck an 11th-hour deal to extend a key debt-refinancing deadline. The stock, which trades under the ticker SAVE in reference to Spirit’s ultralow cost model, climbed 40% to $2.06 in premarket trading. 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.