Fixed AR warnings while executing test suites

Removed warnings when test suites of AR are executed:
```
  /Volumes/kd/projects/kd-rails/activerecord/lib/active_record/associations/builder/collection_association.rb:65: warning: method redefined; discarding old welcome_posts_with_comment_ids
  /Volumes/kd/projects/kd-rails/activerecord/lib/active_record/associations/builder/collection_association.rb:65: warning: previous definition of welcome_posts_with_comment_ids was here
  /Volumes/kd/projects/kd-rails/activerecord/lib/active_record/associations/builder/collection_association.rb:75: warning: method redefined; discarding old welcome_posts_with_comment_ids=
  /Volumes/kd/projects/kd-rails/activerecord/lib/active_record/associations/builder/collection_association.rb:75: warning: previous definition of welcome_posts_with_comment_ids= was here
```