Allow to normalize \Traversable

src/Symfony/Component/Serializer/Encoder/XmlEncoder.php

@@ -305,7 +305,7 @@ class XmlEncoder extends SerializerAwareEncoder implements EncoderInterface, Dec
     {
         $append = true;
 
-        if (is_array($data) || $data instanceof \Traversable) {
+        if (is_array($data) || ($data instanceof \Traversable && !$this->serializer->supportsNormalization($data, $this->format))) {
             foreach ($data as $key => $data) {
                 //Ah this is the magic @ attribute types.
                 if (0 === strpos($key, '@') && is_scalar($data) && $this->isElementNameValid($attributeName = substr($key, 1))) {

src/Symfony/Component/Serializer/Tests/Encoder/XmlEncoderTest.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\Serializer\Tests\Encoder;
 
 use Symfony\Component\Serializer\Tests\Fixtures\Dummy;
+use Symfony\Component\Serializer\Tests\Fixtures\NormalizableTraversableDummy;
 use Symfony\Component\Serializer\Tests\Fixtures\ScalarDummy;
 use Symfony\Component\Serializer\Encoder\XmlEncoder;
 use Symfony\Component\Serializer\Serializer;
@@ -203,6 +204,21 @@ class XmlEncoderTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals($expected, $serializer->serialize($array, 'xml', $options));
     }
 
+    public function testEncodeTraversableWhenNormalizable() {
+        $this->encoder = new XmlEncoder();
+        $serializer = new Serializer(array(new CustomNormalizer()), array('xml' => new XmlEncoder()));
+        $this->encoder->setSerializer($serializer);
+
+        $expected = <<<XML
+<?xml version="1.0"?>
+<response><foo>normalizedFoo</foo><bar>normalizedBar</bar></response>
+
+XML;
+
+        $this->assertEquals($expected, $serializer->serialize(new NormalizableTraversableDummy(), 'xml'));
+
+    }
+
     public function testDecode()
     {
         $source = $this->getXmlSource();