IdPropertyConvention is not working when using other name that is not "_id".

I copy the UnitTest from CSharp77Test below but change _id to _id2, run and see the result.

In MongoDB (currently bug),

{"_id" : ObjectId("xxxxxxxxxxxxxxxxxx"), "_id2" : ObjectId("yyyyyyyyyyyy")}

Expected:

{"_id" : ObjectId("xxxxxxxxxxxxxxxxxx") }

// with out _id2

private class Foo {
public ObjectId _id2

{ get; set; }
public string Name { get; set; }

public string Summary

{ get; set; }

}

[Test]
public void TestSave() {
var server = MongoServer.Create();
var database = server["onlinetests"];
var collection = database.GetCollection<Foo>("csharp77");

var conventions = new ConventionProfile()
.SetIdPropertyConvention(new NamedIdPropertyConvention("_id2"));
BsonClassMap.RegisterConventions(conventions, t => t == typeof(Foo));

collection.RemoveAll();
for (int i = 0; i < 10; i++) {
var foo = new Foo {
_id2 = ObjectId.Empty,
Name = string.Format("Foo-

{0}", i),
Summary = string.Format("Summary for Foo-{0}

", i)
};
collection.Save(foo, SafeMode.True);
var count = collection.Count();
Assert.AreEqual(i + 1, count);
}