Use #next instead of calculating id + 1

The id attribute is not Integer in Mongo, so it fails with something like:

    NoMethodError: undefined method `+' for "5106fc06ee6da1ee44000002":Moped::BSON::ObjectId'`

With #next, it will work with both Integer and String ids, for both AR
and Mongo, returning a different id to test for filtered conditions.