Bug 2833 pt3: Do not respond with HTTP/304 to unconditional requests

Bug 2833 pt3: Do not respond with HTTP/304 to unconditional requests

... after internal revalidation. The original unconditional HttpRequest
was still marked (and processed) as conditional after internal
revalidation because the original (clear) Last-Modified and ETag values
were not restored (cleared) after the internal revalidation abused them.

TODO: Isolate the code converting the request into conditional one _and_
the code that undoes that conversion, to keep both actions in sync.