Storable.add_field_expression() doesn't correctly handle adding a nested String operator that is equivalent to an existing nested Symbol operator. 

This is the second piece of a Symbol-String bug manifesting in a chained Query.where().where(). The first piece is addressed in MONGOID-5066 and its accompanying PR.

The PR is already ready to be reviewed/merged: https://github.com/mongodb/mongoid/pull/4966