Make beginning_of_month return the first of the month in the correct timezone

[mtrudel]

Summary of changes

Takes the time_offset of the beginning of the month into account when computing beginning_of_month. Fixes an issue whereby beginning_of_month called with e.g. 13:45 15 Nov 2018 EST would return 00:00 1 Nov 2018 EST where it should be 00:00 1 Nov 2018 EDT since the Eastern timezone was still in daylight savings on 1 Nov.

Note that other {beginning,end}_of_{day,week,month} functions already do the right thing - the implementation of this method was the outlier.

Checklist

• New functions have typespecs, changed functions were updated
• Same for documentation, including moduledocs
• Tests were added or updated to cover changes
• Commits were squashed into a single coherent commit

[coveralls]

Coverage increased (+2.7%) to 72.561% when pulling 4771fad on mtrudel:master into ea3cd35 on bitwalker:master.

[bitwalker]

Thanks!