When asked to resume a download, libcurl will convert that to HTTP logic
and if then the entire file is already transferred it will result in a
416 response from the HTTP server. With CURLOPT_FAILONERRROR set in that
scenario, it should *not* lead to an error return.
Updated test 1156, added test 1273
Reported-by: Jonathan Watt
Fixes#6740Closes#6753
