fix: support date objects and other non-primitive values

[dominicbarnes]

When sorting by multiple props that include Date objects, you'll end up with incorrect sorting in cases where the underlying date value is the same. The use of === (after checking < and >) will always return false for Date objects, since === compares identity for objects. The result of the final result * sortOrder becomes NaN, which then causes iteration to stop (since the check is looking for === 0).

Just to be clear, this edge-case does not occur when you are only sorting by 1 property, it only comes up for properties following the aforementioned Date objects.

The change here assumes that the returned value will be 0, and will only change to -1 for < and 1 for >. This change is backwards-compatible, and will allow non-primitive values (such as Date) to be handled more gracefully.

This change also includes a test case that was failing before my change, but now passes.

[dominicbarnes]

As an aside, my current workaround is to provide my own custom mapper that looks like this:

function sortMapper(key, value) {
  return value.valueOf()
}

My first proposal was actually going to be to use valueOf() before checking <, > and ===. This actually seemed to work pretty well, but it would have broken backwards-compatibility. This is because anything created with Object.create(null) would throw an exception and anything with a prototype that provided a custom valueOf() would have had their behavior altered. The above change seems to avoid all of that while still getting the job done.