add correct datatime in export

[danieleguido]

The eport function can be test using:

ENV=dev pipenv run python manage.py exportqueryascsv --immediate <user_pk> <solr_query> 

There should ve a .dev.env file in the same folder of manage.py.
where <user_pk> is the primary key of the user table, to check permission; and <solr_query> can be a any valid solr query; --immediate doesn't add the task to celery.