WASHINGTON (Reuters) - Consumer prices were flat in December, pointing to muted inflation pressures that should help give the Federal Reserve room to prop up the economy by staying on its ultra-easy monetary policy path. The Labor Department said on Wednesday its Consumer Price Index was unchanged last month, held back by a drop in gasoline prices. The reading was in line with analysts' expectations in a Reuters poll. Last month, the Fed said it would keep interest rates near zero at least until the jobless rate falls to 6. ...