fails:Array#pack with format '@' ignores whitespace in the format string
fails:Array#pack with format '@' calls #to_str to coerce the directives string
fails:Array#pack with format '@' taints the output string if the format string is tainted
fails:Array#pack with format '@' raises ArgumentError when the format modifier is '_'
fails:Array#pack with format '@' raises ArgumentError when the format modifier is '!'
