global: fix query bugs
global: fix query bugs

* Fixes missing start_date and end_date bug in the file-download
  query.

* Fixes extract_date for python 2.

Signed-off-by: Nicolas Harraudeau <b43b76f088452fbd97430e599a370977191b854c@cern.ch>

inveniosoftware/invenio-stats,inveniosoftware/invenio-stats