Test expansion of array triggered by range assignment

In Opal, the array auto-expansion had a bug that only manifested itself when assigning to a range, but I could find no test on rubyspec that did such an assignment (range + out-of-boundaries), so I'd suggest adding this one (which works fine across MRI/JRuby).

Details on the issue that motivated this PR: https://github.com/opal/opal/pull/504