sync message serializer code for forward-compatibility

Author: xabbuh

Normalizer/MimeMessageNormalizer.php

@@ -66,6 +66,7 @@ public function normalize($object, string $format = null, array $context = [])
         if ($object instanceof AbstractPart) {
             $ret = $this->normalizer->normalize($object, $format, $context);
             $ret['class'] = \get_class($object);
+            unset($ret['seekable'], $ret['cid'], $ret['handle']);
 
             return $ret;
         }