Simplify array_unique

[timmarinin]

Use Array.prototype.indexOf instead of custom key comparator.

This change probably makes function slower due to indexOf probable O(n) timing,
but clarity of the code is more important than the performance in this case.

[amitmerchant1990]

It failed to pass the following case:

array_unique([{a: 'val'}, {b: 'val', c: 'val'}, 'a', 'b', [1,2,3,4], {a: 'val'}, [1,2,3,4], [{a: 'val'}, {b: 'val'}], [{a: 'val'}, {b: 'val'}]]);
// [{a: 'val'}, {b: 'val', c: 'val'}, 'a', 'b', [1,2,3,4], [{a: 'val'}, {b: 'val'}]]

Can you please check?

[atk]

indexOf will only compare the content's indices, not it's contents if those are objects, arrays or NaN. Therefore, this breaks the current functionality.