title: Content Headers Remove fails for string Authorization
body: <p>The following test fails inexplicably:</p>
  
  <pre><code>    [Test]
      public void CrazyAssHttpRequestMessageTest()
      {
          var subject = new HttpRequestMessage()
                            {
                                Method = HttpMethod.Get,
                                Content = new StringContent("some content")
                            };
          subject.Content.Headers.Remove("Authorization");
      }
  </code></pre>
  
  <p>The exceptions is:</p>
  
  <blockquote>
    <p>System.InvalidOperationException : Misused header name. Make sure
    request headers are used with HttpRequestMessage, response headers
    with HttpResponseMessage, and content headers with HttpContent
    objects.</p>
  </blockquote>
  
  <p>Why? Any other header seems to work fine, replace Authorization with something else and all is ok.</p>