title: .NET Web API Post Action returning empty JSON
body: <p>I have a controller inherited from ControllerApi like this (MVC 4)</p>
  
  <pre><code>public HttpResponseMessage&lt;lightUserInfo&gt; Post(LogOnModel model)
  {
      if (ModelState.IsValid)
      {
              if (Membership.ValidateUser(model.UserName, model.Password))
              {
                  MembershipUser user = Membership.GetUser(model.UserName);
                  var userinfo = new lightUserInfo(user);
                  return new HttpResponseMessage&lt;lightUserInfo&gt;(userinfo);
              }
      }
   }
  </code></pre>
  
  <p>I can see the object userinfo is populated fine, and this returns 200 OK to the browser... but the Content is just empty json {} </p>
  
  <p>Here is the client side ajax in CoffeeScript</p>
  
  <pre><code>$.ajax
    url: Meshable.rooturl + "/api/authentication"
    data:   JSON.stringify authenticationDetails    
    dataType: "json"
    type: "POST"
    contentType: 'application/json; charset=utf-8'
    error: (e) -&gt;                     
    success: (data) -&gt;
      #data comes in as empty {}
  </code></pre>