CSHARP-1080: fix unexpected change non id field name in BSON document

https://jira.mongodb.org/browse/CSHARP-1080

I want to serialize following class:
```csharp
public class TestModel_CustomNotId
{
    [BsonElement("id")]
    public ObjectId Id { get; set; }
}
```
I expected Id field to be serialized as "id",
but the result BSON document is containing "_id" field.