By Abhijith Ganapavaram (Reuters) -Spirit Airlines announced on Friday it was assessing options to refinance its 2025 debt maturities amid concerns over its balance sheet, and said its $3.8 billion deal to merge with JetBlue Airways remained intact. Shares of Spirit shot up 20% to $6.83 in morning trade, after losing more than half their value since a Tuesday ruling by District Judge William Young found the proposed deal could threaten competition in the U.S. aviation market and harm ticket prices. If successful, the deal will create the fifth-largest carrier in the United States and help Spir…