Fix support for DKIM signing when forwarding and aliasing is enabled

This partially fix haraka#2774

plugins/aliases.js

@@ -112,6 +112,7 @@ function _alias (plugin, connection, key, config, host) {
             connection.transaction.rcpt_to.pop();
             connection.transaction.rcpt_to.push(toAddress);
         }
+        connection.transaction.notes.forward = true;
     }
     else {
         connection.loginfo(plugin, `alias failed for ${key}, no "to" field in alias config`);

plugins/dkim_sign.js

@@ -355,7 +355,8 @@ exports.get_sender_domain = function (connection) {
         }
     }
 
-    if (!txn.header) return domain;
+    // In case of forwarding, only use the Envelope
+    if (!txn.header || txn.notes.forward) return domain;
 
     // the DKIM signing key should be aligned with the domain in the From
     // header (see DMARC). Try to parse the domain from there.