title: Java - HttpURLConnection - Content-Length isn't shown in request header
body: <p>I create a normal HttpURLConnection-object. I set for example 3 request headers.</p>
  
  <pre class="lang-java prettyprint-override"><code>HttpURLConnection conn = (HttpURLConnection) someURL.openConnection();
  conn.setDoInput(true);
  conn.setRequestMethod("POST");
  conn.setRequestProperty("User-Agent", "some_agent");
  conn.setRequestProperty("Content-Type", "some_content-type");
  conn.setRequestProperty("Content-Length", "some_content-length");
  </code></pre>
  
  <p>I look at setted request headers.
  </p>
  
  <pre><code>System.out.println(conn.getRequestProperties());
  </code></pre>
  
  <p>The header "Content-Length" is never contained in the result. </p>
  
  <pre><code>{User-Agent=[some_agent], Content-Type=[some_content-type]}
  </code></pre>
  
  <p>I am getting also  <code>null</code> if I call <code>conn.getRequestProperty("Content-Length")</code>.</p>
  
  <p>The code works fine but why does "Content-Length" disappear?</p>