decodeCertify not returning signature

The signature portion from the `decoeCertify` _seems_ to just return the whole response/

I can' confirm the exact root cause but can state that the verification of the cetified signature using the AK signer worked with `github.com/google/go-tpm v0.3.1 ` (sample repro [here](https://gist.github.com/salrashid123/50a1904e864acccb596ab49ab2f27fa8))

not with latest@ `github.com/google/go-tpm v0.3.3-0.20210409082102-d3310770bfec` (sample repro [here](https://gist.github.com/salrashid123/c16bbce158e20ec33ef7a21dace7f7d6))

the change was made here:
https://github.com/google/go-tpm/commit/ecde5837c1f82519cd43f90d39399af511db37a3#diff-92947631269f2d84f2baa92d703b215f7843a553a375c945715f669e2e831d7fL1747

---

the diff between working

https://github.com/google/go-tpm/blob/master/tpm2/tpm2.go#L1756

```golang
func decodeCertify(resp []byte) ([]byte, []byte, error) {
var paramSize uint32
var attest, signature tpmutil.U16Bytes
var sigAlg, hashAlg Algorithm

buf := bytes.NewBuffer(resp)
if err := tpmutil.UnpackBuf(buf, ¶mSize); err != nil {
return nil, nil, err
}
buf.Truncate(int(paramSize))
if err := tpmutil.UnpackBuf(buf, &attest, &sigAlg); err != nil {
return nil, nil, err
}
// If sigAlg is AlgNull, there will be no hashAlg or signature.
// This will happen if AlgNull was passed in the Certify() as
// the signing key (no need to sign the response).
// See TPM2 spec part4 pg227 SignAttestInfo()
if sigAlg != AlgNull {
if sigAlg == AlgECDSA {
var r, s tpmutil.U16Bytes
if err := tpmutil.UnpackBuf(buf, &hashAlg, &r, &s); err != nil {
return nil, nil, err
}
signature = append(r, s...)
} else {
if err := tpmutil.UnpackBuf(buf, &hashAlg, &signature); err != nil {
return nil, nil, err
}
}
}
return attest, signature, nil
}
```

and not working

https://github.com/google/go-tpm/blob/v0.3.1/tpm2/tpm2.go#L1600

```golang
func decodeCertify(resp []byte) ([]byte, []byte, error) {
var paramSize uint32
var attest tpmutil.U16Bytes

buf := bytes.NewBuffer(resp)
if err := tpmutil.UnpackBuf(buf, ¶mSize); err != nil {
return nil, nil, err
}
buf.Truncate(int(paramSize))
if err := tpmutil.UnpackBuf(buf, &attest); err != nil {
return nil, nil, err
}
return attest, buf.Bytes(), nil
}
```