fix: TBSCertificate extension parsing.

@indutny, I'm getting this error when parsing a DER cert:

``` bash
Error: Failed to match tag: "objid" at: ["tbsCertificate"]["issuerUniqueID"]["subjectUniqueID"]["extensions"]["extnID"]
```

This PR seems to fix it. The DER in question is at: https://github.com/bitpay/bitcore/blob/master/test/data/x509.der