RSS Feed Image is linked via http even if wiki runs unter https

As a side effect some feed readers do not load the logo (no unencrypted content on an encryptet page)

This seems to be caused by the
com.xpn.xwiki.plugin.feed.FeedPluginApi#getDefaultFeedImage
constructing the absolute URL "manually".

Instead this could be fixed by giving the context an ExternalServletURLFactory, simplifying the code a bit.
Ideally this would not only happen to that method but the complete RSS Feed production process. However it seems the Feed plugin has a lot of entry points ...