title: XElement.Descendants doesn't work with namespace
body: <p>I have a simple XML,</p>
  
  <pre><code>&lt;S xmlns=\"http://schemas.microsoft.com/winfx/2006/xaml/presentation\"&gt;&lt;H&gt;&lt;/H&gt;&lt;/S&gt;
  </code></pre>
  
  <p>I want to find all "H" nodes. </p>
  
  <pre><code>XElement x = XElement.Parse("&lt;S xmlns=\"http://schemas.microsoft.com/winfx/2006/xaml/presentation\"&gt;&lt;H&gt;&lt;/H&gt;&lt;/S&gt;");
  IEnumerable&lt;XElement&gt; h = x.Descendants("H");
  if (h != null)
  {
  }
  </code></pre>
  
  <p>But this code doesn't work. 
  When I remove the namespace from S tag, the code works correctly.</p>