def range
      return nil unless @header['range']
      self['Range'].split(/,/).map {|spec|
        m = /bytes\s*=\s*(\d+)?\s*-\s*(\d+)?/i.match(spec) or
                raise HTTPHeaderSyntaxError, "wrong Range: #{spec}"
        d1 = m[1].to_i
        d2 = m[2].to_i
        if    m[1] and m[2] then  d1..d2
        elsif m[1]          then  d1..-1
        elsif          m[2] then -d2..-1
        else
          raise HTTPHeaderSyntaxError, 'range is not specified'
        end
      }
    end
def range
      return nil unless @header['range']
      self['Range'].split(/,/).map {|spec|
        m = /bytes\s*=\s*(\d+)?\s*-\s*(\d+)?/i.match(spec) or
                raise HTTPHeaderSyntaxError, "wrong Range: #{spec}"
        d1 = m[1].to_i
        d2 = m[2].to_i
        if    m[1] and m[2] then  d1..d2
        elsif m[1]          then  d1..-1
        elsif          m[2] then -d2..-1
        else
          raise HTTPHeaderSyntaxError, 'range is not specified'
        end
      }
    end
Returns an Array of Range objects which represents Range: header field,
 or +nil+ if there is no such header.