TypeNameHandling.Auto works now for object fields

[thatpixelguy]

Fix for this issue:

class Shape { 
  int x; 
  int y; 
}

class Rectangle { 
  int width; 
  int height; 
}

class ThingToSerialize 
{
  // JsonEncoder currently serializes this as a Shape when using TypeNameHandling.Auto
  Shape MyShape = new Rectangle(); 
}