How do I set headers to all responses in Koa.js? : <p>In Express.js I used to have this kind of code:</p>

<pre><code>app.use((req, res, next) =&gt; {
  res.header('Access-Control-Allow-Origin', '*');
  res.header('Access-Control-Allow-Headers', 'Origin, X-Requested-With, Content-Type, Accept');
  res.header('Access-Control-Allow-Methods', 'POST, GET, PUT, DELETE, OPTIONS');
  next();
});
</code></pre>

<p>How do I do the same thing with Koa.js? I need to preset these several http headers for each server response.</p>
