Only enable json converter by default if "json_post" extension is not…

… installed

config/definitions/json_converter.php

@@ -11,7 +11,9 @@
                     ->treatFalseLike(['enabled' => false])
                     ->addDefaultsIfNotSet()
                     ->children()
-                        ->booleanNode('enabled')->defaultTrue()->end()
+                        ->booleanNode('enabled')
+                            ->defaultValue(!\extension_loaded('json_post'))
+                        ->end()
                         ->arrayNode('content_types')
                             ->info('The Content-Type headers on which this converter should act upon.')
                             ->prototype('scalar')->end()