campaign_id is not saved for A/B test mailings

Earlier versions (also 4.6.*) possibly affected.

Let me just explain with an SQL query:
select m.campaign_id from civicrm_mailing m join civicrm_mailing_abtest ab on ab.mailing_id_a = m.id where m.campaign_id is not null;
Empty set (0.00 sec)

Despite setting Campaign while creating an A/B test, the campaign_id is NULL in ALL mailings (a, b and c) related to A/B test.

This makes all information about which campaign a test belongs to lost. Also this makes A/B tests not to show up in mailing reports.