title: The 'valueForKeyPath:' function is not supported
body: <p>Under <em>Array Operations</em> in <a href="https://news.realm.io/news/nspredicate-cheatsheet/" rel="nofollow noreferrer">the Realm NSPredicate cheatsheet</a> it states that we can make a query that targets a specific element in a <code>List</code> via <code>NSPredicate</code>. However, I always get a <em>"The 'valueForKeyPath:' function is not supported."</em> exception when I try the following.</p>
  
  <pre><code>let realm = try! Realm()
  let persons = realm.objects(Person.self)
  let predicate = NSPredicate(format: "orders[0].created == $created")
  persons.filter(predicate.withSubstitutionVariables(["created": Date()]))
  </code></pre>
  
  <p>I'm using Xcode 8.3.3 targeting iOS 9.0 with Realm 2.8.2</p>