Ireland’s Katie Taylor retained her undisputed lightweight world championship with a split decision win over Puerto Rican Amanda Serrano Saturday in a history-making, epic fight at Madison Square Garden. The first women’s bout to headline a fight card at the iconic New York venue in its 140-year history did not disappoint. Brooklyn-based Serrano pummelled Taylor in the fifth round, but the bloodied champion held on and reasserted herself as the pair went toe-to-toe over the final rounds of a breathless contest to edge the decision and remain unbeaten after 21 professional bouts. “I had to dig …