userdefaults should handle deleting and adding keys

[kameghamegha]

Here is an example of an execute statement I haven't been able to put into the userdefaults resource and I believe I should be able to:

execute 'Disable automatic login' do
  command 'defaults delete /Library/Preferences/com.apple.loginwindow '\
          'autoLoginUser'
  only_if 'defaults read /Library/Preferences/com.apple.loginwindow | '\
          'grep "autoLoginUser"'
 end