prepare_range

# File lib/webrick/httpservlet/filehandler.rb, line 119
      def prepare_range(range, filesize)
        first = range.first < 0 ? filesize + range.first : range.first
        return -1, -1 if first < 0 || first >= filesize
        last = range.last < 0 ? filesize + range.last : range.last
        last = filesize - 1 if last >= filesize
        return first, last
      end