title: Send a Cookie header field in relay request
body: <p>It looks like stock relay networklayer does'nt send a cookie header field with his request.
  So I tried to add it by adding Cookie field like this:</p>
  
  <pre><code>Relay.injectNetworkLayer(
    new Relay.DefaultNetworkLayer('/graphql', {
      headers: {
        'Cookie': 'user=thibaut',
      },
    })
  );
  </code></pre>
  
  <p>but still the Cookie field is not present in my post request.
  If I replace 'Cookie' with 'Set-Cookie', IT IS in my post request...</p>
  
  <p>I need my server to use cookies please help ! :)</p>