A suspect was arrested last night in the vicious beating of a cop at a protest Monday, a law-enforcement source said. Jamal Holiday of Manhattan, who turns 20 today, will likely be spending his birthday in jail for assaulting Detective William Sample Jr., according to the source. Sample, meanwhile, was on the mend at home. The 10-year veteran who usually investigates taxi-business crimes, was badly bruised and suffered internal injuries in the attack that Mayor Bloomberg called "outrageous."