Clamp the day of month against 31, not 12, in extract_date

[arpitjain099]

extract_date() in openfda/common.py validates the day of month against 12:

if 0 >= int(month) or 12 < int(month):
  month = "01"
if 0 >= int(day) or 12 < int(day):
  day = "01"

The second bound reads like a copy of the first. The effect is that any day from 13 onward is thrown away and rewritten to the first of the month:

>>> common.extract_date('20121125')
'2012-11-01'
>>> common.extract_date('20121231')
'2012-12-01'

That is roughly 60 percent of all calendar days. AnnotateLabel in openfda/spl/annotate.py feeds the result straight into label['@timestamp'], so SPL effective times land on the wrong day.

Days that overflow a short month do not need this guard either. validate_date() in openfda/spl/fix_date.py already catches day is out of range for month and walks the value back to a real date, which is a better outcome than dropping it to the 1st.

This widens the bound to 31 and leaves the out-of-range case (20121132) behaving exactly as before.

The existing test_extract_date case for day 32 still passes, and I added two cases for valid days past the 12th, which fail on master:

openfda/tests/common_test.py:13: AssertionError
1 failed, 2 passed

With the fix, python3 -m pytest openfda/tests/common_test.py is 3 passed.