#305 Many favicons are only available via HTTP, so leave the URL alone.

Test case: http://www.sheldoncomics.com/index.xml
Actual favicon http://www.sheldoncomics.com/favicon.ico
Requests to https://www.sheldoncomics.com/favicon.ico time out.
Merge pull request #316 from mblakele/305-favicons

#305 Many favicons are only available via HTTP, so leave the URL alone.