Fix feeds without extensions, as FeedBurner’s ones
Fix feeds without extensions, as FeedBurner’s ones

As discussed in pull request : https://github.com/ViennaRSS/vienna-rss/pull/644/files/6760881b6cc77c4cca84dfb171560a75bebd0cde..a78439c850385de7948f2b8a45b47ff01e0b5f4e#r63285685
dak180/vienna,barijaona/vienna-rss,barijaona/vienna-rss,dak180/vienna,tothgy/vienna-rss,barijaona/vienna-rss,lapcat/vienna-rss,dak180/vienna,ViennaRSS/vienna-rss,Eitot/vienna-rss,josh64x2/vienna-rss,josh64x2/vienna-rss,Eitot/vienna-rss,lapcat/vienna-rss,ViennaRSS/vienna-rss,ViennaRSS/vienna-rss,ViennaRSS/vienna-rss,tothgy/vienna-rss,lapcat/vienna-rss,josh64x2/vienna-rss,barijaona/vienna-rss,Eitot/vienna-rss,Eitot/vienna-rss,ViennaRSS/vienna-rss,tothgy/vienna-rss,tothgy/vienna-rss,josh64x2/vienna-rss,dak180/vienna,josh64x2/vienna-rss,lapcat/vienna-rss,barijaona/vienna-rss