title: System.Text.Json.JsonSerializer.Serialize returns empty Json object "{}"
body: <p>Environment: Visual Studio 2019 16.3.8, .NET 3.0.100, .NET Core 3.0 unit test.</p>
  
  <p>All 3 calls below to System.Text.Json.JsonSerializer.Serialize return empty objects: "{}"</p>
  
  <p>I must be doing something wrong ... but I just don't see it?</p>
  
  <pre><code>public class MyObj
  {
      public int myInt;
  }
  
  [TestMethod]
  public void SerializeTest()
  {
      var myObj = new MyObj() { myInt = 99 };
      var txt1 = System.Text.Json.JsonSerializer.Serialize(myObj);
      var txt2 = System.Text.Json.JsonSerializer.Serialize(myObj, typeof(MyObj));
      var txt3 = System.Text.Json.JsonSerializer.Serialize&lt;MyObj&gt;(myObj);
  }
  </code></pre>