title: xhr.setRequestHeader() merging my headers
body: <p>I want to add header to my xhr.</p>
  
  <p>When I use setRequestHeader, what it actually does is adding a value to Access-Control-Request-Headers.</p>
  
  <p>If I code:</p>
  
  <pre><code>xhr.setRequestHeader('key1', 'value1');
  xhr.setRequestHeader('key2', 'value2');
  </code></pre>
  
  <p>What I see in the request header is:
  Access-Control-Request-Headers:accept, key1, key2</p>
  
  <p>What i expect to see instead is:</p>
  
  <p>key1: value1</p>
  
  <p>key2: value2</p>
  
  <p>How can I prevent this merging?
  Thanks!</p>