fix(copy): don't create a row maker on copy

fix(copy): don't create a row maker on copy

A COPY_OUT result has columns, but no names for the columns. This case
must be handled in cur.description (see #235) but we don't need to
handle it in copy. If we did handle it in copy, we would need a column
name fallback, which we forgot to handle, hence the problem in #460.

Close #460.