Remove fold cases and sumAll from CoGroupable.atMostOneFn since folds and sumAll can return 1+ result from 0 input

[dieu]

Hey,

We found the problem with Join and if something before can produce non-empty iterator for empty-iterator, like fold, foldLeft, crazy Semigroup in sumLeft.

While we are joining, we get an iterator from cascading and try to apply everything that's not yet applied to this value, some of the function can produce non-empty iterator even if don't have anything (aka cascading give us empty iterator).

[johnynek]

this is certainly scary.... can you also add one using mapGroup which should also show the bug, since fold is just calling mapGroup...

[dieu]

so, the problem with fold, we doing fold on empty-iterator (when no data on left), but it's legit that fold on empty produce init as result.

[johnynek]

last comment, does only fold trigger it? We don't often use fold, but we do very often use .sum before a join.

I can't believe we don't have test coverage for this case. That's really unfortunate.

[dieu]

last comment, does only fold trigger it? We don't often use fold, but we do very often use .sum before a join.

writing tests for others...