Not normalizing correctly when tt.day<=0

[supernev]

Lua timetable {year=2017, month=12, day=0} means 2017-11-30
But luatz timetable will normalize to 2017-12-01

The same problem when day<=0.

[daurnimator]

Looks like an issue in the year after leap years:

> require"luatz.timetable".new(2016,10,0,0,0,0):normalise()
2016-09-29T00:00:00.000
> require"luatz.timetable".new(2017,10,0,0,0,0):normalise()
2017-10-01T00:00:00.000
> require"luatz.timetable".new(2018,10,0,0,0,0):normalise()
2018-09-30T00:00:00.000

And turns out one of the existing tests is incorrect, which prevented this from being found earlier.

Pushing fix.

[supernev]

Great feedback! Thanks~