title: Query XDocument with xmlns attribute (namespace)
body: <p>I try to query elements from an visual studio *.csproj file. I created a short example to illustrate the problem: </p>
  
  <pre><code>    // Working
      string xml1 = @"&lt;Project ToolsVersion='4.0'&gt;
                        &lt;ItemGroup Label='Usings'&gt;
                          &lt;Reference Include='System' /&gt;
                          &lt;Reference Include='System.Xml' /&gt;
                        &lt;/ItemGroup&gt;
                      &lt;/Project&gt;"; 
      // Not working
      string xml2 = @"&lt;Project ToolsVersion='4.0' xmlns='http://schemas.microsoft.com/developer/msbuild/2003'&gt;
                        &lt;ItemGroup Label='Usings'&gt;
                          &lt;Reference Include='System' /&gt;
                          &lt;Reference Include='System.Xml' /&gt;
                        &lt;/ItemGroup&gt;
                      &lt;/Project&gt;";
  
      XDocument doc = XDocument.Parse(xml2);
  
      foreach (XElement element in doc.Descendants("ItemGroup"))
      {
          Console.WriteLine(element);
      }
  </code></pre>
  
  <p>The string xml1 works fine, xml2 doesn't return anything. The only difference between those strings is xmlns attribute in the document root.</p>
  
  <p>How do i query documents containing xmlns attributes?
  Why is it a problem when a xml document contains an xmlns attribute?</p>