Return full issuer for DN with multiple attribute values

https://redmine.pfsense.org/issues/3542

cert_get_issuer() doesn't always work properly.

e.g. "CN=Some Root CA,OU=Certificates Department,OU=(c) Copyright SomeCorp,O=SomeCorp,C=US"

$v would be array, so $issuer would end up as "CN=Some Root CA,OU=Array,O=SomeCorp,C=US".

cert_get_subject() has the correct logic, so copy that into get_issuer.