Class is persisted without the $type property

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Raven.Client.Document;

namespace ConsoleApplication5
{
    class Program
    {
        private static IList<Person> FakePeople
        {
            get
            {
                var people = new List<Person>
                {
                    new Soldier
                    {

                        Name = "Jane",
                        Equipment = new ReconEquipment
                        {
                            CamoColour = "red",
                            FlareCount = 3
                        }
                    },
                    new Medic
                    {
                        Name = "Joe",
                        Equipment = new HeavyWeaponsEquipment
                        {
                            CamoColour = "blue",
                            SandvichCount = 2
                        }
                    }
                };

                return people;
            }
        }

        static void Main(string[] args)
        {
            var store = new DocumentStore
            {
                Url = "http://localhost:8080/",
                DefaultDatabase = "RhinoNinjas"
            };

            store.Initialize();

            using (var session = store.OpenSession())
            {
                foreach (var person in FakePeople)
                {
                    session.Store(person);
                }
                session.SaveChanges();
            }
        }
    }

    public class Soldier : BasePerson<ReconEquipment>
    {
    }

    public class Medic : BasePerson<HeavyWeaponsEquipment>
    {

    }

    public abstract class BasePerson<TEquipment> : Person where TEquipment : IEquipment
    {
        public TEquipment Equipment
        {
            get { return (TEquipment)base.GetEquipment(); }
            set { base.SetEquipment(value); }
        }
    }

    public abstract class Person
    {
        private IEquipment _equipment;

        public string Name { get; set; }
        public IEquipment GetEquipment() { return _equipment;  }
        public void SetEquipment(IEquipment value) { _equipment = value; }
    }

    public interface IEquipment
    {
        string Name { get; }
        string CamoColour { get; set; }
    }

    public class ReconEquipment : IEquipment
    {
        public string Name { get { return "Recon"; } }
        public string CamoColour { get; set; }
        public int FlareCount { get; set; }
    }

    public class HeavyWeaponsEquipment : IEquipment
    {
        public string Name { get { return "Heavy Metal"; } }
        public string CamoColour { get; set; }
        public int SandvichCount { get; set; }
    }
}