def cover(wobj)
      type = @config['epubversion'] >= 3 ? 'cover' : nil
      s = @epub.cover(type)
      if !s.nil? && !wobj.nil?
        wobj.puts s
      end
    end
def cover(wobj)
      type = @config['epubversion'] >= 3 ? 'cover' : nil
      s = @epub.cover(type)
      if !s.nil? && !wobj.nil?
        wobj.puts s
      end
    end
Write cover file to IO object +wobj+.
 If Producer#config["coverimage"] is defined, it will be used for
 the cover image.