crypto/x509: add test for asn1.Marshal of rsa.PublicKey
Go 1.10 is adding new API MarshalPKCS1PublicKey and
ParsePKCS1PublicKey for converting rsa.PublicKeys.
Even though we'd prefer that users did not, check that
if users call asn1.Marshal and asn1.Unmarshal directly instead,
they get the same results. We know that code exists in the
wild that depends on this.