title: C# WebAPI not serializing dynamic properties correctly
body: <p>I am creating a new C# OData4 Web API with a class named <code>Call</code> that has dynamic properties, which OData 4 allows via "Open Types". I believe I've set everything up and configured it correctly but the serialized response does not include the dynamic properties.</p>
  
  <p>Did I configure something wrong?</p>
  
  <pre><code>public partial class Call 
  {
      public int Id { get; set; }
      public string Email { get; set; }
      public IDictionary&lt;string, object&gt; DynamicProperties { get; }
  }
  </code></pre>
  
  <hr>
  
  <pre><code>public class CallController : ODataController
  {
      [EnableQuery]
      public IQueryable&lt;Call&gt; GetCall([FromODataUri] int key)
      {
          return _context.Call.GetAll();
      }
  }
  </code></pre>
  
  <hr>
  
  <pre><code>public static partial class WebApiConfig
  {
      public static void Register(HttpConfiguration config)
      {
          AllowUriOperations(config);
  
          ODataModelBuilder builder = new ODataConventionModelBuilder();
          builder.ComplexType&lt;Call&gt;();
          var model = builder.GetEdmModel();
  
          config.MapODataServiceRoute(RoutePrefix.OData4, RoutePrefix.OData4, model);    
      }
  
      private static void AllowUriOperations(HttpConfiguration config)
      {
          config.Count();
          config.Filter();
          config.OrderBy();
          config.Expand();
          config.Select();
      }
  }
  </code></pre>