Don't call sort! on ActiveRecord::Associations

The reason is sort! acts same as sort on ActiveRecord::Associations.

  # Source code from ActiveRecord::Associations
  # The @target object is not \loaded until needed. For example,
  #   blog.posts.count
  # is computed directly through SQL and does not trigger by itself the
  # instantiation of the actual post records.

I called sort! on a passed in parameter which is ActiveRecord::Associations and expect it acts like sort! on Array. This makes a bug of that method. I think rails is better to remove sort! from this class’s method list. NoMethodError is much better than this.