apply the order mentioned in the association

This PR fixes https://github.com/rails/rails/issues/6769 . 

However because of this fix 4 tests started failing because now order is being applied. Here are those failign tests https://gist.github.com/neerajdotname/5856608 .

@jonleighton do you think this PR looks ok ? If it looks okay then I will change those four failing tests to remove column name ambiguity. Lemme know. Thanks.