crypto/x509: correct default X.509 version.

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>

src/crypto/x509/x509.go

@@ -126,7 +126,7 @@ type certificate struct {
 
 type tbsCertificate struct {
 	Raw                asn1.RawContent
-	Version            int `asn1:"optional,explicit,default:1,tag:0"`
+	Version            int `asn1:"optional,explicit,default:0,tag:0"`
 	SerialNumber       *big.Int
 	SignatureAlgorithm pkix.AlgorithmIdentifier
 	Issuer             asn1.RawValue