With fuel prices rising, airlines are the last group you'd expect to thrive. But thriving they are. To borrow a quote: It's the economy, stupid. IBD's Transportation-Airline industry group was among the day's top performers, rising 1.7%. The hoopla started when United Airlines (UAUA) said it's seeing stepped-up demand for its highest-margin seats — business class and premium-class international tickets. United reported Tuesday an 18% surge in