title: lengthComputable is always false
body: <p>I've spent an entire day on this... trying and doing research but I couldn't find the right solution for this specific case. I'm doing a very simple task: AJAX HTTP download request with a progress status bar via HTML 5. But lengthComputable is always false, and I can't figure out why! </p>
  
  <p>I have simplified all the code to make it readable:</p>
  
  <p>This is the JavaScript code:</p>
  
  <pre><code>var http = new Object();
  
  http.request = function (){
  
      xmlhttp=new XMLHttpRequest();
  
      xmlhttp.addEventListener("progress", updateProgress, false);
  
      function updateProgress(a){
          console.log(a.lengthComputable)
          if (a.lengthComputable) {
              console.log(a.loaded/a.total*100);
          }
      }
  
      xmlhttp.open("POST",'http://www.name.com/test/test.php',true);
  
      xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
  
      xmlhttp.onreadystatechange=function(){
  
        if(xmlhttp.readyState==4 &amp;&amp; xmlhttp.status==200){
              var response = xmlhttp.responseText;
              console.log(response);
          }
  
        }
      xmlhttp.send();
  
  }
  
  http.request();
  </code></pre>
  
  <p>This is the PHP code:</p>
  
  <pre><code>ob_start();
  
  $i=0;
  $txt='';
  while($i&lt;(1024*1024)){
      $txt.=rand(0, 9);
      ++$i;
  }
  
  header('Access-Control-Allow-Origin: *');
  header('Content-Type: text/plain');
  header('Content-Length: '.strlen($txt));
  header('Accept-Ranges: bytes');
  
  echo $txt;
  
  ob_end_flush(); 
  </code></pre>
  
  <p>I'think this is a server-side problem, maybe some setting that I don't know about Apache / PHP, but this is just a feeling of who doesn't know what to do.</p>