title: How to make availableIncludes work in thephpleague/fractal
body: <p>I'm having trouble implementing Fractal includes. I am trying to include posts with a particular user. </p>
  
  <p>All goes well when I add 'posts' to <strong>$defaultIncludes</strong> at the top of my UserItemTransformer. Posts are included as expected.
  However, posts are NOT included in my json output when I change $defaultIncludes to <strong>$availableIncludes</strong>, even after calling <code>$fractal-&gt;parseIncludes('posts');</code></p>
  
  <p>The problem seems to lie the fact that the method that includes the posts is only called when I use $defaultIncludes. it is never called when I use $availableIncludes.</p>
  
  <p>I'm probably missing something obvious here. Can you help me find out what it is?</p>
  
  <p>This works:</p>
  
  <pre><code>// [...] Class UserItemTransformer
  protected $defaultIncludes = [
      'posts'
  ];
  </code></pre>
  
  <p>This does <strong>not</strong> work:</p>
  
  <pre><code>// [...] Class UserItemTransformer
  protected $availableIncludes = [
      'posts'
  ];
  
  // [...] Class PostsController
  // $fractal is injected in the method (Laravel 5 style)
  $fractal-&gt;parseIncludes('posts');
  </code></pre>