Should _id be ignored in the `flatten` and `simple` parse_methods, instead of only in `dig`?

[ansballard]

Currently, the dig parse method is the only one that explicitly ignores _id, per Line 317. Should that check be added to the other parse methods as well? It currently breaks ES if a field _id is added, so if it can be removed from all parse methods that would be helpful. Happy to make the pull request if this change makes sense.

Thanks!