In Mongoid 5.0 (and earlier) it was possible to make an aggregate query with `out(inline: true)`. After upgrading to 5.2 (tested in 5.2.0 and 5.2.1) a query with such output option will fail:

....map_reduce(map, reduce).finalize(finalize).out(inline: true)
# Mongo::Error::OperationFailure: Invalid collection name: [.....] (73)

Changing to `out(inline: 1)` solves the problem.

I guess this is a non desired behavior and should be changed?