title: Add webapi Header in a DelegatingHandler
body: <p>I am trying to add a header to the request within a Delegating Handler of a webapi project
  like this:</p>
  
  <pre class="lang-cs prettyprint-override"><code>protected async override Task&lt;HttpResponseMessage&gt; SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) 
  {
      var corGuid = CommonBaseController.CreateSafeCorrelationId(string.Empty);
      cor = corGuid.ToString();
      request.Content.Headers.Add("correlationId", cor);
      var response = await base.SendAsync(request, cancellationToken);
  }
  </code></pre>
  
  <p>Once inside the controller I want to be able to get the header like this:
  </p>
  
  <pre><code>var cor = request.Headers["correlationId"];
  </code></pre>
  
  <p>However the header is not there.</p>