fails:String#swapcase returns a new string with all uppercase chars from self converted to lowercase and vice versa
fails:String#swapcase taints resulting string when self is tainted
fails:String#swapcase is locale insensitive (only upcases a-z and only downcases A-Z)
fails:String#swapcase returns subclass instances when called on a subclass
fails:String#swapcase! modifies self in place
fails:String#swapcase! returns nil if no modifications were made
fails:String#swapcase! raises a RuntimeError when self is frozen
