def content_range
    return nil unless @header['content-range']
    m = %r<bytes\s+(\d+)-(\d+)/(\d+|\*)>i.match(self['Content-Range']) or
        raise Net::HTTPHeaderSyntaxError, 'wrong Content-Range format'
    m[1].to_i .. m[2].to_i
  end
Returns a Range object which represents the value of the Content-Range:
header field.
For a partial entity body, this indicates where this fragment
fits inside the full entity body, as range of byte offsets.