startWith: don't use Scheduler.currentThread

More precisely, don't use the implementation of concat that uses
Scheduler.currentThread. This implementation uses Scheduler.immediate,
but only for the concat--startWith's scheduler is still used for
emitting the actual elements prepended to the observable.

Author: rhendric

src/core/linq/observable/startwith.js

@@ -15,5 +15,5 @@
       scheduler = immediateScheduler;
     }
     for(var args = [], i = start, len = arguments.length; i < len; i++) { args.push(arguments[i]); }
-    return enumerableOf([observableFromArray(args, scheduler), this]).concat();
+    return observableConcat.apply(null, [observableFromArray(args, scheduler), this]);
   };

tests/observable/startwith.js

@@ -127,4 +127,29 @@
     );
   });
 
+  test('startWith is unaffected by currentThread scheduler', function () {
+    var scheduler = new TestScheduler();
+
+    var xs = scheduler.createHotObservable(
+      onNext(150, 1),
+      onNext(220, 2),
+      onCompleted(250)
+    );
+
+    var results;
+
+    Rx.Scheduler.currentThread.schedule(null, function () {
+      results = scheduler.startScheduler(function () {
+        return xs.startWith(scheduler, 1);
+      });
+    });
+
+    results.messages.assertEqual(
+      onNext(201, 1),
+      onNext(220, 2),
+      onCompleted(250)
+    );
+
+  });
+
 }());