fails:StringIO#getch increases self's position by one
fails:StringIO#getch returns nil when called at the end of self
fails:StringIO#getch does not increase self's position when called at the end of file
fails:StringIO#getch returns the charactor at the current position
fails:StringIO#getch increments #pos by the byte size of the character in multibyte strings
fails:StringIO#getch returns nil at the end of the string
fails:StringIO#getch StringIO#getch when self is not readable raises an IOError
