Fix $root_mail_recipient to allow arrays

According to the documentation of Mailalias, multiple
recipients should be specified as array, however, this
modules expected $root_mail_recipient to be a string.

manifests/init.pp

@@ -118,7 +118,9 @@ class postfix (
   validate_string($mynetworks)
   validate_string($myorigin)
   validate_string($relayhost)
+  if ! is_array($root_mail_recipient) {
     validate_string($root_mail_recipient)
+  }
   validate_string($smtp_listen)