Have you tried the decoded_content option of HTTP::Response?

I don't know if it actually works in this case because the charset isn't advertised in the HTTP header, but I think it's worth a try.

Update more likely your problem is this:

(iso-8859-5 is the iso name for windows-1251)

The Wikipedia seems to disagree:

Windows-1251 and KOI8-R (or its Ukrainian variant KOI8-U) are much more commonly used than ISO 8859-5, which never really caught on.

And indeed Encode knows of the encoding cp-1251 - maybe use that instead?