title: TypeError: callback.apply is not a function after allowDiskUse
body: <p>I have a collection having 1Million documents... I have passed the option for <code>allowDiskUse</code> and now it is throwing error</p>
  
  <p><code>TypeError: callback.apply is not a function</code></p>
  
  <p>I have searched for this but could get the solution... Please help</p>
  
  <pre><code>const pictures = await Picture.aggregate([
        { $sort: { createdAt: -1 }},
        { $group: {
          _id: { $dateToString: { format: "%Y-%m-%d", date: "$createdAt" } },
          pictures: {
            $push: {
              _id: '$_id',
              image: '$image',
              time: { $dateToString: { format: "%H:%M:%S", date: "$createdAt" } }
            }
          }
        }},
        { $limit: 50 },
        { $project: {
          _id: false,
          createdAt: '$_id',
          pictures: '$pictures'
        }}
      ], { allowDiskUse: true })
  </code></pre>
  
  <p>Mongodb version - 3.6</p>
  
  <p>Mongoose version - 5.0</p>