title: Socket.io gives CORS error even if I allowed cors it on server
body: <p>My node server and client are running on different ports(3001,5347 respectively).
  On client I had used,</p>
  
  <pre><code>var socket = io('http://127.0.0.1:3001');
  </code></pre>
  
  <p>On server I tried all of following one by one</p>
  
  <pre><code>    1) io.set('origins', '*:*');
      2) io.set('origins', 'http://127.0.0.1:5347');
      3) io.set('origins', '*');
      4) io.set('origins', '127.0.0.1:5347');
  </code></pre>
  
  <p>But I'm getting following error:</p>
  
  <blockquote>
    <p>XMLHttpRequest cannot load <a href="http://127.0.0.1:3001/socket.io/?EIO=3&amp;transport=polling&amp;t=1456799439856-4590" rel="noreferrer">http://127.0.0.1:3001/socket.io/?EIO=3&amp;transport=polling&amp;t=1456799439856-4590</a>. A wildcard '*' cannot be used in the 'Access-Control-Allow-Origin' header when the credentials flag is true. Origin 'null' is therefore not allowed access.</p>
  </blockquote>
  
  <p>Note: I'm using express on server and I'm already using cors middleware in following way:</p>
  
  <pre><code>app.use(cors());
  </code></pre>
  
  <p>Where app and cors are instances of express and cors respectively.</p>