(Woops, somehow managed to submit this by accident)

Anyway, imagine we have a truncated entry that has a strong-validator eligible last-modified, and a weak ETag. If we try to resume it, HttpUtil::HasStrongValidators will return true (since the last-modified is true), so we will go ahead and do an if-range... with the weak ETag. This is clearly wrong by the above:
"A client MUST NOT generate an If-Range header field containing an entity-tag that is marked as weak "
... but I am not sure if using the strong last-modified since If-Range would be permitted, either, given the second part:
" A client MUST NOT generate an If-Range header field containing an HTTP-date unless the client has no entity-tag for the corresponding representation ..."

It may be worth asking HTTP spec folks what the intent is here...