fails:String#downcase is locale insensitive (only replaces A-Z)
fails:String#downcase taints result when self is tainted
fails:String#downcase returns a subclass instance for subclasses
fails:String#downcase! returns nil if no modifications were made
fails:String#downcase! raises a RuntimeError when self is frozen
fails:String#downcase! sets the result String encoding to the source String encoding
