Fix 'dateAdded()' getter (#2774)
Fix 'dateAdded()' getter (#2774)

Be homogeneous in field used, to prevent weird behavior in the future. 
aledeg/FreshRSS,marienfressinaud/FreshRSS,aledeg/FreshRSS,FreshRSS/FreshRSS,romibi/FreshRSS,FreshRSS/FreshRSS,aledeg/FreshRSS,FreshRSS/FreshRSS,marienfressinaud/FreshRSS,FreshRSS/FreshRSS,romibi/FreshRSS,marienfressinaud/FreshRSS,marienfressinaud/FreshRSS,aledeg/FreshRSS,Alkarex/FreshRSS,Alkarex/FreshRSS,romibi/FreshRSS,Alkarex/FreshRSS,romibi/FreshRSS,Alkarex/FreshRSS