Populate the server's CertRep with CA certificates before verifying

[omorsi]

This PR is to solve issue #122.

Copying the issue description here to save readers' time.

The following is stated in RFC: PKCS #7: Cryptographic Message Syntax Version 1.5: SignedData type.

certificates is a set of PKCS #6 extended certificates and X.509 certificates. It is intended that the set be sufficient to contain chains from a recognized "root" or "top-level certification authority" to all of the signers in the signerInfos field. There may be more certificates than necessary, and there may be certificates sufficient to contain chains from two or more independent top-level certification authorities. There may also be fewer certificates than necessary, if it is expected that those verifying the signatures have an alternate means of obtaining necessary certificates (e.g., from a previous set of certificates).

IIUC, this means that in SCEP context, it is valid that as a response to PKCSReq message, a SCEP server returns a PKIMessage without adding the CA certificate because there is an alternate means of obtaining necessary certirficates, which is using GetCaCert in this case.

If a SCEP server acted in this valid way, PKCS7 verification will fail in

scep/scep/scep.go

Line 218 in f3adbb7

if err := p7.Verify(); err != nil {

The proposal here is not to remove verification, but to populate the certificates field with the CA certificate obtained by the GetCaCert call before verification.

[omorsi]

@pavolmarko FYI!

[groob]

Nits, but LGTM overall.