title: Can't set Content-Type header
body: <p>I'm having trouble setting the Content-Type on HttpClient.
  I followed along this question: <a href="https://stackoverflow.com/questions/10679214/how-do-you-set-the-content-type-header-for-an-httpclient-request">How do you set the Content-Type header for an HttpClient request?</a>
  But still no luck.</p>
  
  <pre><code>String rcString = JsonConvert.SerializeObject(new RoadsmartChecks() { userguid = user_guid, coords = coordinates, radius = (radius * 100) + "" }, ROADSMART_JSON_FORMAT, JSONNET_SETTINGS);
  HttpClient c = new HttpClient();
  c.BaseAddress = new Uri(BASE_URL);
  c.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "application/json"); //Keeps returning false
  c.DefaultRequestHeaders.TryAddWithoutValidation("Authorization", hash_aes);
  c.DefaultRequestHeaders.TryAddWithoutValidation("Roadsmart-app", Constant.APP_ID);
  c.DefaultRequestHeaders.TryAddWithoutValidation("Roadsmart-user", user_guid);
  c.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
  HttpRequestMessage req = new HttpRequestMessage(HttpMethod.Post, BASE_URL + URL_CHECKS + "/fetch");
  req.Content = new StringContent(rcString);
  await c.SendAsync(req).ContinueWith(respTask =&gt;
  {
      Debug.WriteLine("Response: {0}", respTask.Result);
  });
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/D7IHO.png" alt="Debugger">
  I also tried by using the Flurl library, but it crashes when trying to add the 'Content-Type'.</p>
  
  <pre><code>misused header name content-type
  </code></pre>
  
  <p>So how can I force it so it really adds it?
  Thanks in advance.</p>