Selector.merge!() incorrectly handles nested Hash's containing `$in` and `$nin` operators, where one operator is a String and the other is a Symbol. This PR fixes this issue, along with unit tests that repro the problem behavior in the old code.

This change was originally inspired by a Symbol-String bug manifesting in a chained Query.where().where(). A key cause of this bug was in Selector.merge!(). The second change which fixes the higher level Query.where().where() bug is part of a different module (Storable vs. Selector). I will be submitting that fix + test case(s) in a separate PR.

The PR is already ready to be reviewed/merged: https://github.com/mongodb/mongoid/pull/4965