Fix #616 - fix negative week_of_month/3

[syukronrm]

Summary of changes

Fixes issue #616

Currently week_of_month/3 relying on iso_week/3. But at certain cases iso_week/3 return different year than the year input, it may return the previous or the next year.

Example case for returning week on the previous year.

iex(1)> Timex.iso_week(2021, 1, 1)
{2020, 53}

Thus, it leads to:

iex(3)> Timex.week_of_month(2021, 1, 10)
-51 # should be 1

This is the case for returning the next year.

iex(6)> Timex.iso_week(2019, 12, 31)     
{2020, 1}

Leads to:

iex(7)> Timex.week_of_month(2019, 12, 31)
-46 # should be 6

Checklist

• Tests were added or updated to cover changes
• Commits were squashed into a single coherent commit

[coveralls]

Coverage increased (+0.02%) to 72.141% when pulling eaa2965 on syukronrm:fix-negative-week-of-month into 45424fa on bitwalker:master.

[syukronrm]

I believe this also cover PR #568.

[bitwalker]

The behaviour you specified here is expected - that is how ISO weeks are defined, they don't necessarily perfectly overlap with calendar weeks/year.

[bitwalker]

Sorry, I'm dumb - missed that this wasn't changing the ISO week behavior.