Do not increment the month if month is December

This fixes a date exception we were seeing. It stops us from
incrementing the month from '12' to '13'. In this case also made sure
that the offending method produced the correct output.
Adjust the paydates by a month (to next year) when December
Refactor large if blocks into idiomatic Ruby
Merge remote-tracking branch 'origin/maternity-paternity-variable-interpolation-issue'