title: Update with AddToSet not updating null value with MongoDB C#
body: <p>Using MongoDB, I'm having trouble adding en element to an Array when the array is null.  AddToSet works as expected if I add the item from the console. I am using the official C# driver from 10gen.</p>
  
  <pre><code>var query = Query.EQ("_id", objectId);          
  var itemDoc = item.ToBsonDocument();
  
  //items is an array but currently null
  var update = MongoDB.Driver.Builders.Update.AddToSet("items", itemDoc); // YUNoWork?
  
  //somefield doesn't exist
  var workingUpdate = MongoDB.Driver.Builders.Update.AddToSet("somefield", itemDoc); //works fine
  
  var collection = DataBase.GetCollection&lt;MyObject&gt;(CollectionName);
  
  collection.Update(query, update); // doesn't work
  collection.Update(query, workingUpdate); // works
  </code></pre>
  
  <p>Is this expected behavior? If so, is there a more general way to add items to an array?</p>