Symfony version(s) affected: 4.2

Description
SerializerStamp won't be used in decode.

This will never be true because decodeStamps returns:

array(1) {
      │   [0]=>
      │   object(Symfony\Component\Messenger\Stamp\SerializerStamp)#1012 (1) {
      │     ["context":"Symfony\Component\Messenger\Stamp\SerializerStamp":private]=>
      │     array(0) {
      │     }
      │   }
      │ }

And isset($stamps[SerializerStamp::class]) is checking the key which is 0 here.

This test never checked if the group is actually applied, it just checks if stamp is there and it will always be there, it just won't be used.

How to reproduce
Install messenger with Symfony serializer, add SerializerStamp and see if it is used when you get a response.