Would-be homebuyers and homeowners looking to refinance can give thanks as mortgage rates set new lows this week, although many borrowers with less than perfect credit won't be able to take advantage of the savings that low rates afford.Rates on 30-year fixed-rate mortgages averaged 3.31 percent with an average 0.7 point for the week ending Nov. 21, down from 3.34 percent last week and 3.98 percent a year ago, according to the results of Freddie Mac's weekly Primary Mortgage Market Survey. That's a new low in Freddie Mac records dating to 1971.For 15-year fixed-rate loans, rates averaged 2.63 percent with an average 0.7 point, down 2.65 percent last week and 3.3 percent a year ago. That's also a new record in Freddie Mac records dating to 1991.Five-year Treasury-indexed hybrid adjustable-r...