enable assets when templates are enabled

This puts back the default behavior of Symfony 2.8 where the Asset
component features were implicitly configured with sensible default
values when no explicit configuration was present but the templating
section was enabled.

Author: xabbuh

DependencyInjection/Configuration.php

@@ -43,6 +43,14 @@ public function getConfigTreeBuilder()
         $rootNode = $treeBuilder->root('framework');
 
         $rootNode
+            ->beforeNormalization()
+                ->ifTrue(function ($v) { return !isset($v['assets']) && isset($v['templating']); })
+                ->then(function ($v) {
+                    $v['assets'] = array();
+
+                    return $v;
+                })
+            ->end()
             ->children()
                 ->scalarNode('secret')->end()
                 ->scalarNode('http_method_override')