Don't set if args are blank

[bakunyo]

I want to change the action of named arguments #with_defaults.
followings are example.

# rake file
task :some_task => [:a, :b] do |task, args|
  args.with_defaults({a: 'a', b: 'b'})
  p args.to_hash
end

# execute(now master)
rake some_task[,c]  # => {:a=>"", :b=>"c"}

# execute(expected)
rake some_task[,c]  # =>  {:a=>"a", :b=>"c"}

Rake::TaskArguments are separated by comma, so when I want default value for :a but specify for :b, want to use arguments [,any].

Please let me know if any problems, I will fix my code and resend pull request.

[hsbt]

It's reasonable. I feld [,c] means [nil, c] too. It's better to use default argument.

[bakunyo]

Thank you!