iana.org, www.iana.org and data.iana.org all present a valid TLS
certificate, so let's use it when fetching data to avoid errors in
transit.

Change-Id: I1f295442d24a221fe2b722c4782dceee38b960ec
Reviewed-on: https://go-review.googlesource.com/89415Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>