title: Graph APi: "(#3) Unknown method" error while trying to reply a private message
body: <p>I'm using the Facebook Graph API to reply a direct message.<br>
  I have published my message issuing an HTTP POST request to <code>/CONVERSATION_ID/messages</code> with a valid access token.<br>
  But I receive the following error:  </p>
  
  <pre><code>{
    "error": {
      "message": "(#3) Unknown method",
      "type": "OAuthException",
      "code": 3
    }
  }
  </code></pre>
  
  <p>The same test on the Graph API Explorer returned the same error.<br>
  Can anyone please help me out?</p>