Response Headers support for protocols

[Gozala]

This is followup from #2 where you can find all the details. Below quote is short summary:

Response Headers - Things don't seem all that great here either. From what I understood there is logic branching based on whether url.scheme is http(s) and if so that is where channel is casted to nsIHttpChannel. It was suggested to add another interface like nsIHttpHeadersChannel that would be able to provide HTTP headers, after which we can update earlier mentioned logic branch to support headers on non http(s) protocols as well. As with Request Headers it seems to require C++ changes which makes it more difficult and requires landing it in Firefox. Although necko team seemed 👍 on accepting such changes.

[Gozala]

Migrate to https://bugzilla.mozilla.org/show_bug.cgi?id=1572215