Fix: Do not call callback for Map.prototype.forEach if no map entries exist

[JakeChampion]

Fixes #1299

[gaearon]

Does this work for case like

x.set(42, 'hi')
x.set(43, 'bye')
x.delete(43)

(edit: typo)

?

My intuition is it would have the same problem, calling the callback extra time at the very end because of the deleted item. size wouldn't be 0 but it would still fail.

[JakeChampion]

@gaearon That should work already but just in case I have added a test with your code.

[gaearon]

Apologies for dumb question but could you help me understand why it also works? Since the new check is for .size I would expect the same bug to manifest itself if the last item is marked with undef sentinel, but the map is not empty.

[gaearon]

I think I understand now. If it's the last one, you decrement size. If it's not, then next() will skip the missing items next time.

[JakeChampion]

@gaearon There was still a bug in there, thanks for getting me to look at it even more thoroughly 👍

[JakeChampion]

Screenshots of tests passing in IE 7 and IE 8: