title: MongoDB aggregate framework not keeping what was added with $addFields
body: <p>Field is added but then disappears.
  Here is the code from within the mongo shell:</p>
  
  <pre><code>&gt; db.users.aggregate([{$addFields:{totalAge:{$sum:"$age"}}}])
  { "_id" : ObjectId("5acb81b53306361018814849"), "name" : "A", "age" : 1, "totalAge" : 1 }
  { "_id" : ObjectId("5acb81b5330636101881484a"), "name" : "B", "age" : 2, "totalAge" : 2 }
  { "_id" : ObjectId("5acb81b5330636101881484b"), "name" : "C", "age" : 3, "totalAge" : 3 }
  &gt; db.users.find().pretty()
  { "_id" : ObjectId("5acb81b53306361018814849"), "name" : "A", "age" : 1 }
  { "_id" : ObjectId("5acb81b5330636101881484a"), "name" : "B", "age" : 2 }
  { "_id" : ObjectId("5acb81b5330636101881484b"), "name" : "C", "age" : 3 }
  </code></pre>