title: HttpRequestMessage multiple custom headers overwriting each other
body: <p>I am struggling to understand how to add MULTIPLE headers to the HttpRequestMessage. I mean I get the basics and if I do one header it works but multiple headers and the last one is what sticks and all others are overwritten?</p>
  
  <p>So for example the Accept header will be overwritten but the authorization header will stick.</p>
  
  <pre><code>HttpRequestMessage httpreqmsg = new HttpRequestMessage();
  httpreqmsg.Headers.Add("Accept", "CustomAccept");
  httpreqmsg.Headers.Add("Authorization", "asdfas#%fwqerq@werfds...");
  </code></pre>
  
  <p>Now HttpRequestMessage has an overload with the signature </p>
  
  <pre><code>.Add(string, List&lt;string&gt;) 
  </code></pre>
  
  <p>and that is fine if you have ONE name with multiple values but how do you do multiple headers. TryAddWithoutValidation has the same overloads as above?</p>
  
  <p>TIA</p>
  
  <hr>
  
  <p>Great...so I kinda made a mistake in my post. I didn't think it mattered but it does. I am unit testing a controller and therefore there is no HttpClient object created.</p>