test: fix MongoDB tests for RPC (#4062)

Author: alanpoulain

tests/Fixtures/TestBundle/DataTransformer/RPCOutputDataTransformer.php

@@ -14,6 +14,7 @@
 namespace ApiPlatform\Core\Tests\Fixtures\TestBundle\DataTransformer;
 
 use ApiPlatform\Core\DataTransformer\DataTransformerInterface;
+use ApiPlatform\Core\Tests\Fixtures\TestBundle\Document\RPC as RPCDocument;
 use ApiPlatform\Core\Tests\Fixtures\TestBundle\Dto\RPCOutput;
 use ApiPlatform\Core\Tests\Fixtures\TestBundle\Entity\RPC;
 
@@ -32,6 +33,6 @@ public function transform($object, string $to, array $context = [])
      */
     public function supportsTransformation($object, string $to, array $context = []): bool
     {
-        return $object instanceof RPC && RPCOutput::class === $to;
+        return ($object instanceof RPC || $object instanceof RPCDocument) && RPCOutput::class === $to;
     }
 }