title: XML serialization force full closing tag on null or empty value
body: <p>I have class</p>
  
  <pre><code>public class Testowa
  {
      public string test { get; set; }
  }
  </code></pre>
  
  <p>When I serialize it without value for test, I get</p>
  
  <pre><code>&lt;test/&gt;
  </code></pre>
  
  <p>But I want get </p>
  
  <pre><code>&lt;test&gt;&lt;/test&gt;
  </code></pre>
  
  <p>How I can do it?</p>