Merge strategies produce wrong queries when previous conditions weren't added to top level

Taking the merge strategy example in https://www.mongodb.com/docs/mongoid/master/reference/queries/#merge-strategies and expanding it to operate over a criteria that places the conditions under $and, we get incorrect results:

irb(main):005:0> Band.where(name: {:$in => 42}).in(name: ['a']).override.in(name: ['b'])

=> 

#<Mongoid::Criteria

  selector: {"name"=>{"$in"=>["b"]}, "$and"=>[{"name"=>{"$in"=>["a"]}}]}

  options:  {}

  class:    Band

  embedded: false>

You might expect that `b` would replace `a`, instead the earlier `42` got replaced by `b` and `a` is still there.

If we are not able to come up with a solution, we should at least document this issue.