fails:String#encode raises UndefinedConversionError if the String contains characters invalid for the target     encoding
fails:String#encode replaces invalid characters
fails:String#encode replaces xml characters
fails:String#encode replaces xml characters and quotes the result
fails:String#encode! raises UndefinedConversionError if the String contains characters invalid for the target     encoding
fails:String#encode! replaces invalid characters
fails:String#encode! replaces xml characters
fails:String#encode! replaces xml characters and quotes the result
