Because merging the association scope was removed.

`merge! association.scope(nullify: false)` is expensive but most methods
do not need the merge.

No need `:doc:` for `:nodoc:` classes [ci skip]

Follow up to 5b14129d.

http://edgeapi.rubyonrails.org/classes/ActiveRecord/Attribute.html

[ci skip]

there aren't any instance method defined in this class

* Run actionpack and actioncable tests together
* Demote ac:integration to allow_failures -- it works most of the time,
  but still fails too frequently to rely upon

[ci skip]