Problem Statement/Rationale

When running the aggregate function over a collection, you can send a single aggregation stage without inserting it inside an array. For instance, the following example works properly:

> db.collA.aggregate({$match:{}})

However, it's not possible to run a single aggregation stage over a database:

> db.aggregate({$listLocalSessions: {}})
TypeError: e.some is not a function

Steps to Reproduce

 > db.aggregate({$listLocalSessions: {}})

Expected Results

I would expect either to receive an error explaining that aggregate() is expecting an array, or to execute the aggregation with a single aggregation stage.

Actual Results

We return a misleading error:

 TypeError: e.some is not a function