title: Add Content-Type JSON header in .NET Core
body: <p>I use this <code>return Ok(JsonConvert.SerializeObject(res));</code> for return json in my code and this work fine but content type is <code>text/plain</code></p>
  
  <p>when i use <code>[Produces("application/json")]</code> in my api response be like this:</p>
  
  <pre><code>{\"Value\":\"value1\"}
  </code></pre>
  
  <p>i need use json serialize but also need content type <code>application/json</code>.</p>
  
  <p>please help</p>