title: withCredentials dont belong to request header
body: <p>Using xhr requests in chrome console with and without <code>withCredentials</code> xhrField shows the same request header attributes.</p>
  
  <p>How withCredentials is mapped to request header ?</p>
  
  <p>Typescript call  (Angular 2) :</p>
  
  <pre class="lang-js prettyprint-override"><code>login(username : string, password : string) {
      let data = {username: username, password: password}
      return this.http.post(environment.apiEndpoint + 'login', data, { withCredentials: true })
        .catch((error) =&gt; {
          return Observable.throw(error.json());
        });
    }
  </code></pre>
  
  <p><strong>Note:</strong> It's a cross origin call and I don't recieve cookies even if the server send them (set-cookie header field)</p>