Mortgage rates were mostly unchanged from last week at three-month lows after the Federal Reserve said it would stretch out its purchases of mortgage-backed securities through the end of March.The 30-year fixed-rate mortgage (FRM) averaged 5.04 percent with an average 0.6 point for the week ending Sept. 24, unchanged from a week ago and down from 6.09 percent a year ago, Freddie Mac said in releasing the results of its Primary Mortgage Market Survey.The 15-year FRM this week averaged 4.46 percent with an average 0.6 point, an almost imperceptible change from 4.47 percent last week, but considerably lower than the 5.77 percent registered at this time a year ago.Rates for 30-year fixed-rate loans bottomed at a record low of 4.78 percent in April, largely due to the Federal Reserve's commitme...