Delegate `:each_cons` to `:list`.

Required to write code like https://github.com/Fullscreen/yt/pull/97#issuecomment-58424501
`partnered_channels.includes :viewer_percentages`

`content_owner.partnered_channels` now accepts a chainable `includes`
method that, when called with `:viewer_percentages`, eagerly loads all
the viewer percentages of the partnered channels, so they can be
accessed without N+1 queries.