title: How to append custom fields to HTTP Request header using Request class
body: <p>How to append custom fields to HTTP Request header using Request class in Angular2?
  I found posts which solve similar problems, but they don't use Request class.</p>
  
  <p>My code is the below.</p>
  
  <pre><code>let headers: Headers = new Headers();
  headers.append('foo', 'bar');
  
  let req: any = {
    method : RequestMethod.Get,
    url    : '/test.json',
    headers: headers,
  };
  
  // this.http is Http instance variable.
  this.http.request(new Request(req)).map((res: any) =&gt; res.json());
  </code></pre>
  
  <p>This is very simple code.
  But when executing this, I couldn't find custom fields in HTTP request header using Developer Tools of Google Chrome.</p>
  
  <p>Referenced: <a href="https://angular.io/docs/js/latest/api/http/index/Request-class.html" rel="nofollow">https://angular.io/docs/js/latest/api/http/index/Request-class.html</a></p>
  
  <hr>
  
  <p>It's possible to reproduce this issue.
  The below code should get XHR 400 errors, but it's not problem. But note that the request header don't have custom fields.</p>
  
  <p><a href="http://plnkr.co/edit/arCFx69V9H1Cl0pJXBzO?p=preview" rel="nofollow">http://plnkr.co/edit/arCFx69V9H1Cl0pJXBzO?p=preview</a></p>