Fixed GetChanges: Already visited items in object-graph were ignored and have not been set to null if unchanged or deleted
Lets say one was unchanged, so if it is more then once availible in the object graph they will appeared different.
 the first one would be set to null, because !visitationHelper.IsVisited(trackableRef) , if the same entity appears in another node of object graph, it will not been nulled, because visitationHelper.IsVisited(trackableRef) == true
So for those we have to check (TrackingState == unchanged or deleted )and if so then set to null.