function ensureDecimalSep(pattern) {
  return (pattern.indexOf(DECIMAL_SEP) !== -1)
      ? pattern : pattern.replace(LAST_ZERO_RE, '$1' + DECIMAL_SEP + '$2');
}
Helper function for parser.
Ensures that `pattern` (e.g #,##0.###) contains a DECIMAL_SEP, which is necessary for further
parsing. If a pattern does not include one, it is added after the last ZERO (which is the last
thing before the `posSuf` - if any).