fails:IO#codepoints returns an Enumerator when passed no block
fails:IO#codepoints yields each codepoint
fails:IO#codepoints yields each codepoint starting from the current position
fails:IO#codepoints raises an error if reading invalid sequence
fails:IO#codepoints does not change $_
fails:IO#codepoints raises an IOError when self is not readable
fails:IO#codepoints calls the given block
