There are certain cases where you can end up with a collision with an implicit
key (array index) if your explicit key prop is a number. They should use
different namespaces. Therefore I wrap explicit keys in curlies and implicit
array indices in brackets.
I added a simple test case, but another case came up on the mailing list. Where
undefined entries in an array actually results in an entry and therefore an
implicit key.
Sebastian Markbage
61b38b9f05