Downloading bridge descriptors is not working, we get server descriptors

In base.py we have

   # Get bridge descriptors.
    data = downloader.get_server_descriptors(\
            descriptor_type = "bridge-server-descriptor 1.0",
            document_handler = DocumentHandler.DOCUMENT).run()
    with open("%s/bridge-descriptors" % (path), "wb") as fd:
        sd_str = ''.join(map(str, data))
        sd_str_output = sd_str.encode('utf-8')
        fd.write(sd_str_output)

However, we end up with server descriptors in the bridge-descriptors file, e.g. for

• https://metrics.torproject.org/rs.html#details/FD7D298F7CDF916285CE664D10B370D152A8A506