Auto eager loading broken for has_and_belongs_to_many associations that use the uniq option

[jturkel]

Consider the following models:

class Post < ActiveRecord::Base
  has_and_belongs_to_many :authors, uniq: true
end

class Author < ActiveRecord::Base
  has_and_belongs_to_many :posts, uniq: true
end

The uniq option is ignored by Rails when eager loading Post#authors.

Note this is only a problem in Rails 3 since the uniq option was removed in Rails 4 (in favor of using a lambda to customize the association scope).