Uses key.to_s as a fallback in #localized_string if no value is given
Sets default fallback for #localized_string to nil

If nil is passed as a value, NSBundle#localizedStringForKey:value:table: will return key if a localized string isn't found.
Thanks @clayallsopp for the notice
Merge pull request #181 from jeroenj/localized_string

Uses key.to_s as a fallback in #localized_string if no value is given