Worldborder set command doesn't take current motion into account

When performing a worldborder set command while the border is moving, the resulting motion will assume that the world border had already reached its stopping point.

For example, type

/worldborder set 100

then

/worldborder set 10 60

then

/worldborder set 100 60

Instead of reversing its movement back out to 100, it will jump in to 10 and start moving outward.