Fix date filter in search.

This change is for https://www.pivotaltracker.com/story/show/45933843.

After merge we should not be seeing Heading 2851 here https://www.gov.uk/trade-tariff/search?utf8=%E2%9C%93&q=trade_tariff&t=water&button=&day=18&month=3&year=2013&country=.

But it should still be seen before 31-12-2006 (its validity end date). https://www.gov.uk/trade-tariff/search?utf8=%E2%9C%93&q=trade_tariff&t=water&button=&day=18&month=3&year=2005&country=