This commit fixes a bug in WC_Customer_Download_Data_Store::get_downloads() that made impossible to change the order in which the query returned the results. This method accepts the arguments `order_by` and `order` but it was ignoring them and always using the default values ('permission_id' and 'ASC' respectively).
One of the assertions of the WC_Customer_Download_Data_Store::get_downloads() test method was modified to make sure the code now works.
This bug was introduced by commit a443419.
* fix user id for download file
* fix access expires for download when not set
‚NULL‘ trigger a 0000-00-00 00:00:00
as date in the database column. In order to write a real Null to the
database, the quotations must be deleted