ocsp: fix default value of Version in ResponseData

ASN.1 specification of `ResponseData` is:

    ResponseData ::= SEQUENCE {
          version              [0] EXPLICIT Version DEFAULT v1,
          responderID              ResponderID,
          producedAt               GeneralizedTime,
          responses                SEQUENCE OF SingleResponse,
          responseExtensions   [1] EXPLICIT Extensions OPTIONAL }

Where `Version` is:

    Version ::= INTEGER { v1(0) }

Invalid default value results in a presence of a field that otherwise
should not be present.

See: https://github.com/openssl/openssl/issues/1297
Fixes: golang/#16321

Change-Id: Ibb065a67624e1877cfbe62e0483d781d783f74f0
Reviewed-on: https://go-review.googlesource.com/24841
Reviewed-by: Adam Langley <agl@golang.org>

ocsp/ocsp.go

@@ -114,7 +114,7 @@ type basicResponse struct {
 
 type responseData struct {
 	Raw              asn1.RawContent
-	Version          int           `asn1:"optional,default:1,explicit,tag:0"`
+	Version          int           `asn1:"optional,default:0,explicit,tag:0"`
 	RawResponderName asn1.RawValue `asn1:"optional,explicit,tag:1"`
 	KeyHash          []byte        `asn1:"optional,explicit,tag:2"`
 	ProducedAt       time.Time     `asn1:"generalized"`