BUG: when parsing partial date, use sane defaults for missing fields GH2618

dateutil uses the current date to fill in missing fields, this
is desirable for year and month, but will cause an exception
when code is run on the 31st day of a month, and trying to parse
"2000 2" as feb 2000, because feb doesn't have 31 days.

So we default to current year and month, but always the first of the month.