Fix query with nested array in Active Record

`User.where(id: [[1,2],3])` was equal to `User.where(id:[1, 2, 3])`
in Rails 4.1.x but because of some refactoring in Arel this stopped
working in 4.2.0. This fixes it in Rails. Fixes #16580.
[Dan Olson & Cristian Bica]

/cc @sgrif @matthewd @DanOlson @chancancode 
That should be out of the scope of this PR , and should be handled more generally elsewhere  if we actually want to do this (we don't)