implemented review feedback from @russcam, unneccessary cast + only set scroll to _doc order if the user did not supply a sort of their own

Author: Mpdreamz

src/Nest/Document/Multiple/ScrollAll/ScrollAllObservable.cs

@@ -29,7 +29,8 @@ public ScrollAllObservable(
 			this._connectionSettings = connectionSettings;
 			this._scrollAllRequest = scrollAllRequest;
 			this._searchRequest = scrollAllRequest?.Search ?? new SearchRequest<T>();
-			this._searchRequest.Sort = DocOrderSort;
+			if (this._searchRequest.Sort == null)
+				this._searchRequest.Sort = DocOrderSort;
 			this._searchRequest.RequestParameters.Scroll(this._scrollAllRequest.ScrollTime.ToTimeSpan());
 			this._client = client;
 			this._compositeCancelTokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);

src/Nest/Document/Multiple/ScrollAll/ScrollAllRequest.cs

@@ -50,7 +50,7 @@ public ScrollAllRequest(Time scrollTime, int numberOfSlices)
 		}
 		public ScrollAllRequest(Time scrollTime, int numberOfSlices, Field routingField) : this(scrollTime, numberOfSlices)
 		{
-			((IScrollAllRequest)this).RoutingField = routingField;
+			this.RoutingField = routingField;
 		}
 	}