SkipWhile(): don't init to default value (#695)

Rx.NET/Source/src/System.Reactive/Linq/Observable/SkipWhile.cs

@@ -85,7 +85,6 @@ namespace System.Reactive.Linq.ObservableImpl
                     : base(observer)
                 {
                     _predicate = predicate;
-                    _index = 0;
                 }
 
                 public override void OnNext(TSource value)