minor #17780 [TwigBridge] Symfony 3.1 forward compatibility (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[TwigBridge] Symfony 3.1 forward compatibility

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/pull/17743#discussion_r52718344
| License       | MIT
| Doc PR        |

Commits
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2991639 [TwigBridge] Symfony 3.1 forward compatibility

src/Symfony/Bridge/Twig/Extension/YamlExtension.php

@@ -12,6 +12,7 @@
 namespace Symfony\Bridge\Twig\Extension;
 
 use Symfony\Component\Yaml\Dumper as YamlDumper;
+use Symfony\Component\Yaml\Yaml;
 
 /**
  * Provides integration of the Yaml component with Twig.
@@ -40,7 +41,7 @@ class YamlExtension extends \Twig_Extension
         }
 
         if (defined('Symfony\Component\Yaml\Yaml::DUMP_OBJECT')) {
-            $dumpObjects = (int) $dumpObjects;
+            return $dumper->dump($input, $inline, 0, is_bool($dumpObjects) ? Yaml::DUMP_OBJECT : 0);
         }
 
         return $dumper->dump($input, $inline, 0, false, $dumpObjects);