Fix firefox event issue related to Button tag

[benadam11]

This is the same issue as #118 but it also applies to button on FF

Example of issue: https://84r5vjm2yl.codesandbox.io/ (FF only)

[clauderic]

Actually upon further investigation I actually don't think this is the same issue as #118. I believe the issue you're pointing out isn't actually specific to Firefox, and rather is just the default implementation for shouldCancelStart, see the default implementation here: https://github.com/clauderic/react-sortable-hoc/blob/master/src/SortableContainer/defaultShouldCancelStart.js#L10

Most people want their buttons to still be interactive and not cause a sort event to begin, so by default react-sortable-hoc prevents sort events from being triggered on button elements. You can fork the default shouldCancelStart implementation and define your own that you can pass as the shouldCancelStart prop of your enhanced SortableContainer component.

I'm going to be reverting this PR