Always call refresh on parent of drag and drop operation.

STUD-2048

We need to refresh the parent even when drag and drop is all within the same parent element. Otherwise, children of parent will be wrong until the next fetch operation occurs.

@dan-f @benmcmorran @polesye please review
Not to reviewer: this wasn't necessary for the test to pass (only checking a certain method was called), but the logic of the unit test was confusing because we intended to drag unit-1 AFTER unit-2.