Changed the line zipped.first.first

It used to be !zipped.first.first. The change must have happened when squashing
two commits together.
Anyway, !zipped.first.first works, whereas zipped.first.first doesn't.
Refactoring chunk
Added a test for calling chunk with one argument

The behaviour is described in the ruby documentation. However there is no spec
in rubyspec for it.
Modified chunk so that it can also deal with 1 arg