Fix scroll-spy events leaking after component unmount
Fix scroll-spy events leaking after component unmount

* If scroll-spy is enabled and the component is unmounted the scroll
  events will still hang around and attempt to set state.
* The design should probably be changed to not use globals (a service
  instead perhaps?), though this is a quick fix.

producthunt/react-scroll,daviferreira/react-scroll,slashtu/react-scroll,fisshy/react-scroll,vanderlin/react-scroll,ntdb/react-scroll,wearableintelligence/react-scroll,synapsestudios/react-scroll,producthunt/react-scroll,dandelany/react-scroll