fails:String#unpack with format '@' ignores whitespace in the format string
fails:String#unpack with format '@' calls #to_str to coerce the directives string
fails:String#unpack with format '@' raises a TypeError when passed nil
fails:String#unpack with format '@' raises a TypeError when passed an Integer
fails:String#unpack with format '@' raises an ArgumentError when the format modifier is '_'
fails:String#unpack with format '@' raises an ArgumentError when the format modifier is '!'
fails:String#unpack with format '@' moves the read index to the byte specified by the count
fails:String#unpack with format '@' implicitly has a count of zero when count is not specified
fails:String#unpack with format '@' has no effect when passed the '*' modifier
fails:String#unpack with format '@' positions the read index one beyond the last readable byte in the String
fails:String#unpack with format '@' raises an ArgumentError if the count exceeds the size of the String
