MONGOID-5030 Short-circuit trivial queries where $in is given an empty array

[jamis]

Summary

When Mongoid.allow_short_circuit_queries == true, any top-level $in query condition that evaluates to an empty array will cause the query to be short-circuited, returning an empty array without sending the query to the database.

Mongoid.allow_short_circuit_queries = true
Document.in([]) #=> empty array, no database query issued

Mongoid.allow_short_circuit_queries = false
Document.in([]) #=> empty array, one database query issued

The Mongoid.allow_short_circuit_queries setting defaults to false.