title: HttpClient PostAsJsonAsync method not works correctly(Serialization not works correctly)
body: <p>I have a Web api controller and i used HttpClient PostAsJsonAsync() method; my object(Employee) is inherited from base class(Person) but after the object is posted to the Api, The Object type is changed(unserialized correctly);
  Here is my classes:- 
  <strong>Before Sending, the Object type is Employee
  After Sending,  the Object type is Person</strong>
  please see the attachments</p>
  
  <pre><code>public class Person
  {
     public Guid Id { get; set; }
     public String Name { get; set; }
  }
  
  public class Employee : Person
  {
      public int Age { get; set; }
  }
  
  public class CreateEmployeeRequest 
  {
      public Person Person { get; set; }
  }
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/G3Y4V.png" alt="enter image description here"><img src="https://i.stack.imgur.com/jiORm.png" alt="enter image description here"></p>