only calculate next_id when we need a new method defined
rewrite tests to have no side-effects on `include_root_in_json`.
`ActiveRecord::Base.include_root_in_json` is `false` by default.

Closes #9459.

The PR #6597 unified the configuration for `include_root_in_json`
in AM and AR to `false`.
Later on with the refactoring commit: e030f26 the value in AR was
set to `true` but I think this was not on purpose.

With this commit both AM and AR will have the same configuration
for `include_root_in_json`, which is `false`.