Make the code more straightforward

Author: shana

src/GitHub.App/Services/ModelService.cs

@@ -146,7 +146,7 @@ public ITrackingCollection<IPullRequestModel> GetPullRequests(ISimpleRepositoryM
             var keyobs = GetUserFromCache()
                 .Select(user => string.Format(CultureInfo.InvariantCulture, "{0}|{1}|pr", user.Login, repo.Name));
 
-            TrackingCollection<IPullRequestModel> col = null;
+            var col = new TrackingCollection<IPullRequestModel>();
 
             var source = Observable.Defer(() => keyobs
                 .SelectMany(key =>
@@ -160,7 +160,7 @@ public ITrackingCollection<IPullRequestModel> GetPullRequests(ISimpleRepositoryM
                 .Select(Create)
             );
 
-            col = new TrackingCollection<IPullRequestModel>(source);
+            col.Listen(source);
             return col;
         }
 

src/GitHub.Exports.Reactive/Collections/TrackingCollection.cs

@@ -98,7 +98,6 @@ public TrackingCollection(IObservable<T> source,
             IScheduler scheduler = null)
             : this(comparer, filter, scheduler)
         {
-            this.source = source;
             Listen(source);
         }