fix time_tag helper and update tests to not rely on dynamically generated values

  Before `time_tag Date.today` will return `<time
  datetime="2013-02-14T00:00:00+00:00">February 14, 2013</time>`.
  This commit fixes time_tag helper to use RFC-3339 full-date format
  (Y-m-d) in datetime attribute.
document setting custom datetime attribute for time_tag helper