For Enumerables containing 0 or 1 items, the #each_in_parallel method
was improperly calling the block without an argument, then calling it
again properly but not returning the result of the block.
The #each method returns the Enumerable that was it was called on,
rather than the value of the block. This needs to be #map instead, to
actually return an array of the one or zero values. The tests weren't
catching this because they were effectively passing `identity` as the
block, nullifying the distinction between #each and #map.