fix: send Autocrypt header in MDNs

Otherwise MDNs are attributed to address-contacts
rather than key-contacts.

Author: link2xt

src/mimefactory.rs

@@ -528,7 +528,7 @@ impl MimeFactory {
             Loaded::Message { msg, .. } => {
                 msg.param.get_bool(Param::SkipAutocrypt).unwrap_or_default()
             }
-            Loaded::Mdn { .. } => true,
+            Loaded::Mdn { .. } => false,
         }
     }