rgw: Do not send a Content-Length header on a 304 response

rgw: Do not send a Content-Length header on a 304 response

We tell the client that the content has not changed. If we
send a Content-Length header RFC2616 describes that the client
MUST use that new value:

"If a cache uses a received 304 response to update a cache entry,
the cache MUST update the entry to reflect any new field values
given in the response."

Therefor we should not send a Content-Length header

Fixes: #15119
Signed-off-by: Wido den Hollander <wido@42on.com>