Wrong throttling implementation

[mareksuscak]

Update throttling is implemented incorrectly on this line. It not only doesn't throttle the provided function at all (in fact a result of calling _spy is passed into throttle). It also creates a new function every time a scroll event is fired. This results in a huge performance hit and increased memory usage due to the number of allocations.

[mareksuscak]

Submitted a PR: https://github.com/makotot/react-scrollspy/pull/128