See https://mullvad.net/en/help/dns-over-https-and-dns-over-tls/
The Mullvad DoH servers appear to only speak HTTP/2 and
the use of a non-nil DialContext in the http.Transport
means that ForceAttemptHTTP2 must be set to true to be
able to use them.
Signed-off-by: Nahum Shalman <nahamu@gmail.com>