# File lib/aws/s3/request.rb, line 151
      def canonicalized_headers
        x_amz = headers.select{|name, value| name.to_s =~ /^x-amz-/i }
        x_amz = x_amz.collect{|name, value| [name.downcase, value] }
        x_amz = x_amz.sort_by{|name, value| name }
        x_amz = x_amz.collect{|name, value| "#{name}:#{value.to_s.strip}" }.join("\n")
        x_amz == '' ? nil : x_amz
      end