[ChangeLog][QtQml] Fix JavaScript Array.pop() not updating the internal array length correctly

While the length property was reporting the correct value, the internal array
length was out-of-sync.

Task-number: QTBUG-35979

Change-Id: I68820a349cf1ce88c6aabc6a2301a8a861018a10
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>