bug #31026 [Serializer] Add default object class resolver (jdecool)

This PR was squashed before being merged into the 4.2 branch (closes #31026).

Discussion
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[Serializer] Add default object class resolver

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

The commit 1d8b5af introduce a BC break because before that commit the `extractAttributes` the `$object` can be a string which contain the fully qualified name of an object.

To fix the BC break and preserve the new feature, I suggest to create a default object class resolver if it is not set by the developer.

Commits
-------

dd5b8f1 [Serializer] Add default object class resolver

Author: fabpot

src/Symfony/Component/Serializer/Normalizer/ObjectNormalizer.php

@@ -43,7 +43,10 @@ public function __construct(ClassMetadataFactoryInterface $classMetadataFactory
         parent::__construct($classMetadataFactory, $nameConverter, $propertyTypeExtractor, $classDiscriminatorResolver, $objectClassResolver, $defaultContext);
 
         $this->propertyAccessor = $propertyAccessor ?: PropertyAccess::createPropertyAccessor();
-        $this->objectClassResolver = $objectClassResolver;
+
+        $this->objectClassResolver = $objectClassResolver ?? function ($class) {
+            return \is_object($class) ? \get_class($class) : $class;
+        };
     }
 
     /**
@@ -63,7 +66,7 @@ protected function extractAttributes($object, $format = null, array $context = [
         $attributes = [];
 
         // methods
-        $class = $this->objectClassResolver ? ($this->objectClassResolver)($object) : \get_class($object);
+        $class = ($this->objectClassResolver)($object);
         $reflClass = new \ReflectionClass($class);
 
         foreach ($reflClass->getMethods(\ReflectionMethod::IS_PUBLIC) as $reflMethod) {

src/Symfony/Component/Serializer/Tests/Normalizer/ObjectNormalizerTest.php

@@ -1043,6 +1043,30 @@ public function denormalize($propertyName, string $class = null, string $format
         $this->assertArrayHasKey('foo-Symfony\Component\Serializer\Tests\Normalizer\ObjectDummy-json-bar', $normalizer->normalize(new ObjectDummy(), 'json', ['foo' => 'bar']));
     }
 
+    public function testDefaultObjectClassResolver()
+    {
+        $normalizer = new ObjectNormalizer();
+
+        $obj = new ObjectDummy();
+        $obj->setFoo('foo');
+        $obj->bar = 'bar';
+        $obj->setBaz(true);
+        $obj->setCamelCase('camelcase');
+        $obj->unwantedProperty = 'notwanted';
+
+        $this->assertEquals(
+            [
+                'foo' => 'foo',
+                'bar' => 'bar',
+                'baz' => true,
+                'fooBar' => 'foobar',
+                'camelCase' => 'camelcase',
+                'object' => null,
+            ],
+            $normalizer->normalize($obj, 'any')
+        );
+    }
+
     public function testObjectClassResolver()
     {
         $classResolver = function ($object) {