title: mongodb mongoose js update not inserting new field
body: <p>I am trying to insert a new field 'description' with a findOneAndUpdate statement using Mongoose js. It is not working. The code works fine when the field already exists and updates it with the given value. According to the Mongodb documentation 
  $set should create this field automatically if it does not exist. </p>
  
  <p>Any ideas appreciated.</p>
  
  <pre><code>Course.findOneAndUpdate({'_id': CourseId},{$set:{'description':courseDescription}},function(err, course) {
          if (err)
              res.jsonp({response:'err'});
  
          res.jsonp(course);
      });
  </code></pre>