Phenylalanine Dehydrogenase Catalyzed Reductive Amination of 6-(1′,3′-Dioxolan-2′-YL)-2-Keto-Hexanoic Acid to 6-(1′,3′-Dioxolan-2′-YL)-2S-Aminohexanoic Acid with Nadh Regeneration and Enzyme and Cofactor Retention