`init` function unnecessarily runs `_sortNodeDown` on all nodes

[ericbstie]

The init function only needs to run _sortNodeDown on all non-leaf nodes, e.g. i < heapArray.length / 2 (see https://stackoverflow.com/a/18742428/8594404). I think this was the intention, because of the Math.floor call; just forgot to divide by 2 ;)

heap-js/src/Heap.ts

Line 428 in c6bce4c

for (let i = Math.floor(this.heapArray.length); i >= 0; --i) {

[ignlg]

Perfectly addressed by #664. Closing the issue. Thanks!