The default version of an X.509 certificate is v1, which is encoded on
the wire as a zero.

Fixes #13382.

Change-Id: I5fd725c3fc8b08fd978ab694a3e2d6d2a495918b
Reviewed-on: https://go-review.googlesource.com/20548Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>