title: How to add headers to a httprequestmessage
body: <p>I would like to add headers to a HttpRequestMessage. Problem is HttpRequestHeaders has no constructor, HttpHeaders is abstract, ... .
  Is there any way we can use these classes somewhere in our code ( I could use a  <code>List&lt;Tuple&lt;string, List&lt;string&gt;&gt;</code> but does not look nice at all)</p>