Add support for numeric months in MwTimeIsoFormatter

"n" and "m" are numeric months (without/with leading zero).

But "xj..." and "xm..." are extended formats and need to be
excluded here. These are currently not used in the "dmy ..."
formats, that's why I currently can't add more tests.

Change-Id: Iba32128e8de716562e29abe5b374fb19786d25ba