ArgumentError not raised when Array arg is missing about slop

[leejarvis]

Thanks for the great bug report!

I've spent a bit of time thinking about this. I think I could just remove this code, which would avoid setting the default value when a flag is used without any argument. However, perhaps there's an argument for supporting --flag where the flag expects an argument, allowing it to behave like optparse does:

OptionParser.new do |o|
  o.on("--list [x,y,z]") do |items|
    puts "items: #{items.inspect}"
  end
end.parse!

# ruby run.rb --list
#=> items: nil

Not really sure why anyone would want that, but if the code above is removed from Slop, then it's straight up no longer possible to provide a flag without any argument where a default is provided. It would also be a breaking change

What do you think?