Upstream: avoid closing client connection in edge case.

If proxy_cache is enabled, and proxy_no_cache tests true, it was previously
possible for the client connection to be closed after a 304. The fix is to
recheck r->header_only after the final cacheability is determined, and end the
request if no longer cacheable.

Example configuration:

proxy_cache foo;
proxy_cache_bypass 1;
proxy_no_cache 1;

If a client sends If-None-Match, and the upstream server returns 200 with a
matching ETag, no body should be returned to the client. At the start of
ngx_http_upstream_send_response proxy_no_cache is not yet tested, thus cacheable
is still 1 and downstream_error is set.

However, by the time the downstream_error check is done in process_request,
proxy_no_cache has been tested and cacheable is set to 0. The client connection
is then closed, regardless of keepalive.

src/http/ngx_http_upstream.c

@@ -2861,6 +2861,11 @@ ngx_http_upstream_send_response(ngx_http_request_t *r, ngx_http_upstream_t *u)
         ngx_http_file_cache_free(r->cache, u->pipe->temp_file);
     }
 
+    if (r->header_only && !u->cacheable && !u->store) {
+        ngx_http_upstream_finalize_request(r, u, 0);
+        return;
+    }
+
 #endif
 
     p = u->pipe;