Minor fixes in WorkingRangeViewController.swift

Summary:
Using more suitable type cast.
Using guard statement.

- [x] All tests pass. Demo project builds and runs.
- [ ] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/98

Differential Revision: D4056145

Pulled By: rnystrom

fbshipit-source-id: 537fccc781747f835155bfe8ccac45a1e8cdb037

Example/IGListKitExamples/ViewControllers/WorkingRangeViewController.swift

@@ -27,10 +27,9 @@ class WorkingRangeViewController: UIViewController, IGListAdapterDataSource {
         var arr = [Int]()
         while arr.count < 20 {
             let int = Int(arc4random_uniform(200)) + 200
+            guard !arr.contains(int) else { continue }
             // only use unique values
-            if !arr.contains(int) {
-                arr.append(int)
-            }
+            arr.append(int)
         }
         return arr
     }()
@@ -51,7 +50,7 @@ class WorkingRangeViewController: UIViewController, IGListAdapterDataSource {
     //MARK: IGListAdapterDataSource
 
     func objects(for listAdapter: IGListAdapter) -> [IGListDiffable] {
-        return data as [NSNumber]
+        return data as [IGListDiffable]
     }
 
     func listAdapter(_ listAdapter: IGListAdapter, sectionControllerFor object: Any) -> IGListSectionController {