Spirit Airlines, an icon of budget air travel that reshaped the industry, has filed for bankruptcy protection after years of mounting losses, a failed merger and more demanding consumer tastes.The carrier early Monday announced a prearranged deal with its bondholders including $300 million in debtor-in-possession financing. It said that vendors, aircraft lessors will not be impaired.The airline said it expects to continue operating normally "throughout this prearranged, streamlined chapter 11 process" and s ...