modify fetch to pass on getFavicon parameter

[Issue 218](https://github.com/owncloud/news/issues/218)

second PR for this issue, code worked but was never used, except in the tests :/ Unlike the tests the implemented code used Fetcher::fetch to call FeedFetcher::fetch which did not use the second parameter which untfortunately resulted in $getFavicon=true in FeedFetcher::fetch