title: symfony/config arrayNode prototype addDefaultsIfNotSet
body: <p>I'm sorry to say I can't find a way to add a default value to a "symfony/config": "2.6.4" ConfigurationInterface !</p>
  
  <p>What's wanted is this type of config :</p>
  
  <pre><code>X:
      Y:
          - test
          - testing
  </code></pre>
  
  <p>With default to :</p>
  
  <pre><code>X:
      Y:
          - test
  </code></pre>
  
  <p>By "default" I mean : if Y config branch is not set on the read configuration file, the $processor->processConfiguration should add it (and it does ! If I remove the ->prototype...)</p>
  
  <p>Here is my code :</p>
  
  <pre class="lang-php prettyprint-override"><code>class Definition implements ConfigurationInterface
  {
      /**
       * {@inheritdoc}
       */
      public function getConfigTreeBuilder()
      {
          $treeBuilder = new TreeBuilder();
          $rootNode = $treeBuilder-&gt;root("X");
          $rootNode
              -&gt;children()
                  -&gt;arrayNode("Y")
                      -&gt;addDefaultsIfNotSet()
                      -&gt;info("Multiple values can be used")
                      -&gt;cannotBeEmpty()
                      -&gt;addDefaultIfNotSet()
                      -&gt;defaultValue(array("test"))
                      -&gt;prototype("scalar")
                          -&gt;validate()
                          -&gt;ifNotInArray(array("test", "testing"))
                              -&gt;thenInvalid("Invalid value %s")
                          -&gt;end()
                      -&gt;end()
                  -&gt;end()
              -&gt;end()
          ;
  
          return $treeBuilder;
      }
  }
  </code></pre>
  
  <p>Sure, I read this question <a href="https://stackoverflow.com/questions/11937710/using-the-symfony2-configuration-class-how-do-i-define-an-array-node-whose-chil">Using the Symfony2 configuration class, how do I define an array node whose children don&#39;t have keys?</a></p>
  
  <p>My current code implements this way as you can read, but it does not work, my code throws :</p>
  
  <pre><code>[Symfony\Component\Config\Definition\Exception\InvalidDefinitionException]
    -&gt;addDefaultsIfNotSet() is not applicable to prototype nodes at path "X.Y"
  </code></pre>