Fixes: Transforming a comma only array to an empty array fails

[gandazgul]

Ok, I realized my mistake on the previous PR (#12), this new code will match modern browser behavior in IE8 by replacing empty array elements with explicit undefined. Using void 0 instead of undefined to guarantee the value doesn't change (Undefined can be reassigned). Handles the case of multiple trailing commas correctly (replace all with undefined except for the last one which is discarded.)

thanks

[sophiebits]

[,,] is still slightly different from [undefined, undefined] -- in the former, the keys are not set at all (and '0' in arr would be false) and in the latter, they are set to undefined. I'm not going to merge a transformation to undefined but if it's working for you, please continue using it!