I think there is a problem with r657443 (and thus I assume r645813): I applied the r657443 patch to my copy of the 2.2.8 official release source and it doesn't work right. The problem is that the change to mod_proxy_http.c checks c->keepalives for a value, but c->keepalives is filled out by ap_http_header_filter(), which isn't called until later. So, c->keepalives is always 0 at this point. Which also means that the "number of keepalives" message in the ap_log_rerror() msg isn't going to mean anything, but at this point it never reaches it.
Also, I "patched the patch" to manually set c->keepalives to 1 for testing so that I could test out this r657443 patch. However, what happened was that Apache would kick back an error page to the client, instead of just closing the connection. So, no improvement. Has this patch been tested and verified anywhere?
