fixes #6696 - API v2 - specify the key in which parameters would be wrapped rather than passing model name

specify `:host` as key rather than passing class name `Host::Base` which was incorrectly resolved to `params[:base]`
re-committed. Added `@request.env['HTTP_ACCEPT']` here as well as `password_confirmation` to 'api/v2/users_controller.rb' since api v2 tests pass `password_confirmation`