def search_upper_header(hdr)
      hdr.class.known_headers.each do |nh, bindings|
        return nh if bindings.check?(hdr)
      end

      nil
    end
def search_upper_header(hdr)
      hdr.class.known_headers.each do |nh, bindings|
        return nh if bindings.check?(hdr)
      end

      nil
    end
Search a upper header for +hdr+
 @param [Header::Base] hdr
 @return [void]
 @yieldparam [Header::Base] found upper header