Fix response for array of plain objects

For an array of plain objects it won't create the choosen root key in the json.

For example:
```
class User 
  def model_name
    'user'
  end
end
@user = User.new(first_name: 'Luke', last_name: 'Skywalker')
render_for_api :public, :json => [@user]
```
Previous output:
{"records":[{"record":{"first_name":"Luke","last_name":"Skywalker"}}]}

New output:
{"users":[{"user":{"first_name":"Luke","last_name":"Skywalker"}}]}