isUnderThreshold returns false for a card that didn't moved

[tiangolo]

isUnderThreshold() doesn't check if the variable this.thresholdAmount exists before computing and ends up returning false when the card hasn't moved, when should return true if anything (if the card hasn't moved yet this.thresholdAmount is undefined).

My use case: I'm using the plug-in with a list of cards (not a "stack"). To do that I create a stack with just one card for each card. I modified the plug-in to use only dragright and dragleft to use only horizontal dragging and allow scrolling. But when I scroll the page, a dragend event is fired and since the card didn't move at all there is no this.thresholdAmount and the value of calling isUnderThreshold() is false. That triggers the onDestroy handler (in my case removing the card) while the card didn't even moved.

I'm submitting a PR to solve this.