Handle multiple zero-offset Scrollspy elements.

Created by: neoeno

When the first two elements tracked by Scrollspy have an offset.top of 0, Scrollspy will flip between activating their respective nav events on every scroll event. This causes a visual 'flicker', and heavily degrades scroll performance.

Examples: Minimal example. Example with nested sections.

I saw this happen in a system of nested sections positioned hard against the top of the document, e.g.

<section id="animals">
  <section id="dogs">
    Content
  </section>
</section>

The bug is that Scrollspy's check to see if it's at the end of the array of sections uses !arr[index + 1]. This misses the case where arr[index + 1] does exist and is zero.

This commit explicitly checks the array bounds. A unit test is included, but I'm not familiar with bootstrap's testing approach so feedback is appreciated.

(Thanks go to @theroux and @tnguyen14!)