This method returns an Array of the receiving MutableBuffer's substrings that were separated by the separator string. separator may be any string, including the null string. If the null string is used, an Array containing each character of the MutableBuffer is returned. If the target MutableBuffer starts with the separator, the first Array item will be a null string. If the MutableBuffer ends with a separator, no extra null string item will be added. If separator isn't specified, any line-end indicator is honored.