parse iso8601 dates so that partial dates work

Ruby’s DateTime::iso8601 method doesn’t give reliable results when given partial dates
2014 will give you todays date but at 20:14
2014-03 will give you 2014-01-03 instead of 2014-03-01
If no results in search use now as modified date.
Only use response_set’s updated_at date so search and displayed results are consistent
Merge pull request #960 from theodi/bugfix-improve-date-filtering

improve date filtering