Refactor HasManyThroughAssociation to inherit from HasManyAssociation. Association callbacks and <association>_ids= now work with hm:t. Closes #11516 [rubyruy]


git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@9230 5ecf4fe2-1ee6-0310-87b1-e25e094e27de