title: d3.json() fail but jQuery $.getJSON() work correctly with same URL and php code
body: <p>i work with d3.js and CodeIgniter and i have a problem when i get json data with d3.json
  when i run this code :</p>
  
  <pre><code>var url = "http://probe.dev/draw/windRose?station=vp2&amp;sensors=wind&amp;Since=2012-10-15T00:00:00&amp;StepUnit=DAY&amp;StepNbr=6;
  d3.json(url, function(d) {
      console.log(d); // print NULL in console
  }
  </code></pre>
  
  <p>but it works with</p>
  
  <pre><code>$.getJSON(url, function(d) {
      console.log(d); // print my data object correctly
  }
  </code></pre>
  
  <p>my php code is :</p>
  
  <pre><code>&lt;?php
  @ob_end_clean();
   header('Content-Type: "application/json"');
   header('Content-Disposition: attachment; filename="data.json"');
   header("Content-Transfer-Encoding: binary");
   header('Expires: 0');
   header('Pragma: no-cache');
   header("Content-Length: ".strlen($data));
    exit($data);
  </code></pre>
  
  <p>I don't understand why d3.json do not work ?</p>