Make isArrayLike check more precise to avoid false positives

[acusti]

Fixes #1438

Update isArrayLike to check for:

• a numeric length
• confirm that it has the length - 1 key set (even if it is undefined)
• if length is 0, then check that it doesn’t have any other properties other than length

The “accepts an array-like” tests in List.ts and Set.ts both used an array-like object that has a length of 3 and a value set at index 1. This fails the updated logic, which verifies instead that there is a value set at the final index, as opposed to any arbitrary possible index. So, I updated those tests to have a value set at the final index in the array-like objects.

I also expanded the Seq.ts “accepts an array-like” test to include:

• an example with a plain object that has a length but is not array-like, and so shouldn’t be converted to a List ({ length: 1, foo: 'bar' })
• an example with a plain object that looks like an empty array and should be converted to a List ({ length: 0 })