Upstream: allow to intercept responses with status 300.

This fixes an omission made in 9e7926763f87 where all 3XX statuses
were allowed for "error_page".

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 45e2eb7..4183d9d 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -1660,7 +1660,7 @@
 
     /* rc == NGX_OK */
 
-    if (u->headers_in.status_n > NGX_HTTP_SPECIAL_RESPONSE) {
+    if (u->headers_in.status_n >= NGX_HTTP_SPECIAL_RESPONSE) {
 
         if (r->subrequest_in_memory) {
             u->buffer.last = u->buffer.pos;