Go Slices don't report length as an own property

[mstoykov]

Original report to the k6 project.

The original issues used to work in k6 until the es6 merge at which point it stopped but it only works for []interface{} which is what happens when you just unmarshal an array from json (the original use case). Even before the es6 branch this didn't work for anything else, but I would argue that ... they should especially as if you access length it is obviously there.

A simple reproducible test

        vm := New()
        s := []interface{}{2, 3, 4}
        vm.GlobalObject().Set("s", vm.ToValue(s))
        fmt.Printf("%#v\n", vm.ToValue(s))
        v, err := vm.RunString(`s.hasOwnProperty("length")`)
        if err != nil {
                t.Fatal(err)
        }
        fmt.Println(v) // this should be true but it's false unless you go back to 1f8a2ffe986ff105e41a12cc8b82857c3e1c2d44

For the record this only works because this resolves to using objectGoSlice not objectGoSliceReflect, which also doesn't work. And no actual combination seems to work currently :(.