Update BsonMapper.Serialize.cs to correct Serialize interface

[tivial19]

This changes will help to Serialize interfaces correct. Now it is working like this:
public interface IItemData
{
bool Auto { get; set; }
string Name { get; set; }
int Value { get; set; }
}

public class ItemData : IItemData
{
public bool Auto { get; set; }
public string Name { get; set; }
public int Value { get; set; }
public double DD { get; set; }
}

List Data = new List() { new ItemData(true, 87, "First", 111.111), new ItemData(false, 43, "Second", 222.222) };

using (var db = new LiteDatabase(DBname))
{
var Col = db.GetCollection(CollectionName);//here i can set interface type
Col.InsertBulk(Data);// and here i cannot set interface type and programm use type of object (BsonMapper.Serialize.cs - line 183(184))
}
And i get doc with all 4 fields Auto, Value, Name and DD, but i need only 3 like in interface.