asn1: fix marshalling of empty optional RawValues

This fixes creation of X509 certificates with
RSA keys. (Broken by e5ecc416f2fd)

R=agl
CC=golang-dev
https://golang.org/cl/4553052

src/pkg/asn1/marshal.go

@@ -458,11 +458,12 @@ func marshalField(out *forkableWriter, v reflect.Value, params fieldParameters)
 		return marshalField(out, v.Elem(), params)
 	}
 
-	if v.Type() == rawValueType {
-		rv := v.Interface().(RawValue)
-		if rv.Class == 0 && rv.Tag == 0 && len(rv.Bytes) == 0 && params.optional {
+	if params.optional && reflect.DeepEqual(v.Interface(), reflect.Zero(v.Type()).Interface()) {
 		return
 	}
+
+	if v.Type() == rawValueType {
+		rv := v.Interface().(RawValue)
 		err = marshalTagAndLength(out, tagAndLength{rv.Class, rv.Tag, len(rv.Bytes), rv.IsCompound})
 		if err != nil {
 			return
@@ -471,10 +472,6 @@ func marshalField(out *forkableWriter, v reflect.Value, params fieldParameters)
 		return
 	}
 
-	if params.optional && reflect.DeepEqual(v.Interface(), reflect.Zero(v.Type()).Interface()) {
-		return
-	}
-
 	tag, isCompound, ok := getUniversalType(v.Type())
 	if !ok {
 		err = StructuralError{fmt.Sprintf("unknown Go type: %v", v.Type())}

src/pkg/asn1/marshal_test.go

@@ -45,6 +45,10 @@ type printableStringTest struct {
 	A string "printable"
 }
 
+type optionalRawValueTest struct {
+	A RawValue "optional"
+}
+
 type testSET []int
 
 func setPST(t *time.Time) *time.Time {
@@ -102,6 +106,7 @@ var marshalTests = []marshalTest{
 			"7878787878787878787878787878787878787878787878787878787878787878",
 	},
 	{ia5StringTest{"test"}, "3006160474657374"},
+	{optionalRawValueTest{}, "3000"},
 	{printableStringTest{"test"}, "3006130474657374"},
 	{printableStringTest{"test*"}, "30071305746573742a"},
 	{rawContentsStruct{nil, 64}, "3003020140"},