encoding/asn1: fix returned type for an Object Identifier

encoding/asn1: fix returned type for an Object Identifier

Unmarshal/Marshal/Unmarshal was not idempotent as the Object Identifier
type was not returned through the interface. The limit case OID = 0
returns an error. The zero OID is 0.0

A test is fixed to use the Object Identifier type.
Other related test are added.

Fixes #11130

Change-Id: I15483a3126066c9b99cf5bd9c4b0cc15ec1d61ca
Reviewed-on: https://go-review.googlesource.com/113837
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>