Support nested operators in abilities

[johnnyshields]

This PR enables using nested operators such as $in in the matchers for abilities.

      @ability.can :foo, MongoidProject, title: { '$in' => %w[Sir Lord] }

Without this PR, you get the following error in the above case:

NoMethodError: undefined method `$in' for "Sir":String

          condition_match?(subject.send(name), value)
                                  ^^^^^