def range
    return nil unless @header['range']

    value = self['Range']
    # byte-range-set = *( "," OWS ) ( byte-range-spec / suffix-byte-range-spec )
    #   *( OWS "," [ OWS ( byte-range-spec / suffix-byte-range-spec ) ] )
    # corrected collected ABNF
    # http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-19#section-5.4.1
    # http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-19#appendix-C
    # http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-19#section-3.2.5
    unless /\Abytes=((?:,[ \t]*)*(?:\d+-\d*|-\d+)(?:[ \t]*,(?:[ \t]*\d+-\d*|-\d+)?)*)\z/ =~ value
      raise Net::HTTPHeaderSyntaxError, "invalid syntax for byte-ranges-specifier: '#{value}'"
    end

    byte_range_set = $1
    result = byte_range_set.split(/,/).map {|spec|
      m = /(\d+)?\s*-\s*(\d+)?/i.match(spec) or
              raise Net::HTTPHeaderSyntaxError, "invalid byte-range-spec: '#{spec}'"
      d1 = m[1].to_i
      d2 = m[2].to_i
      if m[1] and m[2]
        if d1 > d2
          raise Net::HTTPHeaderSyntaxError, "last-byte-pos MUST greater than or equal to first-byte-pos but '#{spec}'"
        end
        d1..d2
      elsif m[1]
        d1..-1
      elsif m[2]
        -d2..-1
      else
        raise Net::HTTPHeaderSyntaxError, 'range is not specified'
      end
    }
    # if result.empty?
    # byte-range-set must include at least one byte-range-spec or suffix-byte-range-spec
    # but above regexp already denies it.
    if result.size == 1 && result[0].begin == 0 && result[0].end == -1
      raise Net::HTTPHeaderSyntaxError, 'only one suffix-byte-range-spec with zero suffix-length'
    end
    result
  end
Returns an Array of Range objects which represent the Range:
HTTP header field, or +nil+ if there is no such header.