Upstream: NGX_HTTP_CLIENT_CLOSED_REQUEST no longer reset to 0.

The NGX_HTTP_CLIENT_CLOSED_REQUEST code is allowed to happen after we
started sending a response (much like NGX_HTTP_REQUEST_TIME_OUT), so there
is no need to reset response code to 0 in this case.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index c4e0c64..8590f4e 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -3405,6 +3405,7 @@
 
     if (u->header_sent
         && rc != NGX_HTTP_REQUEST_TIME_OUT
+        && rc != NGX_HTTP_CLIENT_CLOSED_REQUEST
         && (rc == NGX_ERROR || rc >= NGX_HTTP_SPECIAL_RESPONSE))
     {
         rc = 0;