The decision by Moody’s Investors Service to bump the District’s credit rating from “junk bond” to “investment-grade” is further evidence that the city is cleaning up its act. But it apparently takes more than that to convince the hardest-working man in show business. At his 9:30 Club show last Thursday, James Brown had just finished his standard rap about the universal need for peace, love, and unity when he turned to a rather more parochial theme: “I tell you one thing,” Brown said. “We’ve got to stick together to clean up this city, because it’s a rotten mess….It’s a hell of a mess to see D.C. look so filthy.”