include_root_in_json setting retrieval does not respect hierarchy

ActiveModel has a knob called include_root_in_json (documented in https://api.rubyonrails.org/classes/ActiveModel/Serializers/JSON.html) which controls the structure of the as_json method output. This knob is supported by Mongoid but its values are not being read correctly.

Test case: https://github.com/p-mongo/tests/blob/master/mongoid-app/lib/tasks/test_4865.rake

Output:

Mongoid: false

Foo: false

foo: false

Sub: false

sub: false

Mongoid -> true

Mongoid: true

Foo: false

foo: true

Sub: false

sub: true

Foo -> true

Mongoid: false

Foo: true

foo: false

Sub: true

sub: false

Sub -> true

Mongoid: false

Foo: false

foo: false

Sub: true

sub: false

The class level value is not influenced by the global Mongoid value, and the instance value is not influenced by the class level value.