Treat http:// and https:// guid as being the same when fetching

Treat  and https:// guid as being the same when fetching

This means that if the same blog post shows up both under http:// and
https:// (for example when somebody changes their URL from http to
https, but it still contains the old posts), we will treat them as being
the same and not fetch a second copy of it.

We handle both http->https (common) and https->http (would probably
indicate a misconfiguration) scenarios.