fixes #6991 - API v2 - support unwrapped POST/PUT parameters even though it's not documented

The goal of this PR is the preempt any possible bugs from API consumers that try to POST/PUT unwrapped params.
@domcleal, yes, as a preemptive steps, we could add _ids in all places, if we are fully supporting unwrapped POST/PUT parameters, even the non-documented parameters like _ids. If I remember correctly, I think certain _ids were added in this PR to make tests pass.