Allow parse_root_in_json's format option to be inherited

We ran into a bug w/ Her that caused the `parse_root_in_json` format option not to be shared from a parent class.

Given this setup:

```
# remote_model.rb
class RemoteModel
  include Her::Model

  parse_root_in_json true, format: :active_model_serializers
end

# user.rb
class User < RemoteModel
end
```

While User will inherit the `true` value of `parse_root_in_json`, it will not inherit the `:active_model_serializers` format option. This PR refactors the predicate methods to allow for this.