title: HttpWebRequest response is always text/html when calling cross domain URL
body: <p>I am using HttpWebRequest to get json result from a cross domain (<a href="https://www.facebook.com/plugins/post/oembed.json/?url=posturl" rel="nofollow noreferrer">https://www.facebook.com/plugins/post/oembed.json/?url=posturl</a>). I have set ContentType and Accept to application/json, but I always get a text/html response. Expected result is JSON.</p>
  
  <pre><code>string result = string.Empty;
  var request = (HttpWebRequest)WebRequest.Create("https://www.facebook.com/plugins/post/oembed.json/?url=xxxxxx");
  
  request.ContentType = "application/json; charset=utf-8";
  request.Accept = "application/json";
  request.Method = "POST";                
  
  var response = (HttpWebResponse)request.GetResponse();
  using (var streamReader = new StreamReader(response.GetResponseStream()))
  {
      result = streamReader.ReadToEnd();
  }
  </code></pre>