By ANNE D'INNOCENZIO, AP Retail Writer NEW YORK (AP) — Shares of online mattress pioneer Casper Sleep Inc. popped Thursday in their debut on the New York Stock Exchange. But the New York-based company still faces challenges to prove to investors it can profitably sell mattresses. Its shares closed at $13.50, up nearly 13% above the $12 IPO price. The stock opened at $14.50 and traded as high as $15.85.