Fix of Mapping provider to allow use of inheritance

[PospisilBohumir]

.Update() and .Delete() don't work with inherited entities e.g. for entities:
public abstract class Customer
{
public long Id { get; set; }
public string Name { get; set; }
}

public class CustomerA : Customer
{
    public string Info { get; set; }
}

....
the Update call:
context.Set().Update(o => new CustomerA
{
Info = "X",
});

will throw exception. Similar problem occurred in case of delete
This fix should solve this problems