[Bug Fix] Fix morphToMany's pivot update and delete bug.

This is related to previous PR https://github.com/laravel/framework/pull/4288 and issue https://github.com/laravel/framework/issues/3693

Without setting the morphClass, we can't do $foo->relations->find(1)->pivot->update()/delete(). Refer to https://github.com/laravel/framework/issues/3693 for the cause.

I didn't use the solution proposed in https://github.com/laravel/framework/issues/3693 as the morphClass should not be load as a model attribute, and should not be included in saved attributes.