cis -Stilbene and (1α,2β,3α)-(2-Ethenyl-3-methoxycyclopropyl)benzene as Mechanistic Probes in the Mn III (salen)-Catalyzed Epoxidation: Influence of the Oxygen Source and the Counterion on the Diastereoselectivity of the Competitive Concerted and Radical-Type Oxygen Transfer