Bug 777685: Use UTF-8 encoding on all email parts, to work around buggy email clients.

Bugzilla/BugMail.pm

@@ -390,6 +390,8 @@ sub _generate_bugmail {
         $email->content_type_set($parts[0]->content_type);
     } else {
         $email->content_type_set('multipart/alternative');
+        # Some mail clients need same encoding for each part, even empty ones.
+        $email->charset_set('UTF-8') if Bugzilla->params->{'utf8'};
     }
     $email->parts_set(\@parts);
     return $email;