onSlidingStart only starting if onSlide is defined

[jmosner]

An onSlidingStart function supplied as a prop is only called, if the onSlide prop is also present.

As a substitute I was able to supply an empty function for onSlide, and onSlidingStart is executed as it should.

[3DJakob]

I can confirm this issue as well! I created a pull request to fix the issue here #29