Fix a bug with the timezone offset format when unparsing dates.
ISO 8601 syntax requires the timezone offset format to be "+HH" or "+HHMM" or "+HH:MM" . Currently the unparser uses something like the second format, but the HH portion turns into H instead of 0H if the hour is less than 10 (e.g. +200 instead of +0200). This commit fixes this.