Re-compute Range response content offset after an FTP response was adapted.

Re-compute Range response content offset after an FTP response was adapted.

Moved HTTP-only offset calculation code to the general ServerStateData code so
that [converted-to-HTTP] FTP responses can use it. FTP code computes the range
offset on its own earlier, but we must REcalculate in case the response is
adapted into a response with a different (or no) content range.