This test has been added because IdFieldSelector has one responsability : check if a given field is an 'id' field or not
With Jackson, this field can be annotated with @JsonProperty("_id") or @Id.

It's up to ObjectIdUpdater to check if this field can be considered as an ObjectId : https://github.com/bguerout/jongo/pull/113/files#L3R39

This check is tested here : https://github.com/bguerout/jongo/pull/113/files#L6R119 , Jongo will not allow an Integer to be annotated with @Id