tags: restful test fix

* Fixes tests where records were created by using dates as strings. Now
  date column values are set with  datetime objects (fix for MySQL 5.6).

* Tests for tags REST API passed.

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>