mailing report: unique opens detail view inaccurate

when viewing a mailing report, there is a stat for unique opens and total opens. the stats on that overview page are accurate, but if you click to view the detailed report, the unique opens listing does not actually show you unique values.

this is due to a flaw in the query (CRM_Mailing_Event_BAO_Opened::getRows()) – if the distinct flag is passed, it groups by the event queue id AND the timestamp, where it should only group by the event queue id.