PR raised here: https://github.com/mongodb/mongoid/pull/5034

The native MongoDB interface for $unset uses a Hash object rather than an array. Therefore, for consistency, Mongoid should allow a Hash to be the argument to `.unset`.

{ $unset: { <field1>: "", ... } }

According to [MongoDB's docs](https://docs.mongodb.com/manual/reference/operator/update/unset/):

> The specified value in the $unset expression (i.e. "") does not impact the operation.

So we should simply take the Hash's keys, even if the value is nil or false.