crypto/x509: if a parent cert has a raw subject, use it.

crypto/x509: if a parent cert has a raw subject, use it.

This avoids a problem when creating certificates with parents that
were produce by other code: the Go structures don't contain all the
information about the various ASN.1 string types etc and so that
information would otherwise be lost.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5453067