Allegedly broke and drug-addled, Keith Jackson was hardly a model father-to-be — telling his three-months-pregnant girlfriend, “I hope you spit out a boy.” Then, early Thursday, Jackson pumped a single bullet into girlfriend Bridget Shenk’s chest, killing her and their unborn child in the hallway outside her Staten Island apartment, police said. Arrested at the scene, Jackson, 43, has been charged with murder and gun possession, police said.