Update JSONObject.java

I don't know if anyone wants this (or whether it violates JSON standards) but I've found that when I use JSONObject.accumulate I want to force an array object.  This saves from having to write checking code on the client side to see if the item is an object or an array.  You can just iterate over it the same way (because sometimes collections only have one item).  

In any case, this mod does it, but I suspect this function may have been written this way on purpose.  Anyway, I love and depend on this library so thanks for keeping it maintained.