update ip_hash to "backup" option

src/http/modules/ngx_http_upstream_ip_hash_module.c

diff --git a/src/http/modules/ngx_http_upstream_ip_hash_module.c b/src/http/modules/ngx_http_upstream_ip_hash_module.c
index 9604d5f..1d13481 100644
--- a/src/http/modules/ngx_http_upstream_ip_hash_module.c
+++ b/src/http/modules/ngx_http_upstream_ip_hash_module.c
@@ -140,7 +140,7 @@
 
     /* TODO: cached */
 
-    if (iphp->tries > 20 || iphp->rrp.peers->number == 1) {
+    if (iphp->tries > 20 || iphp->rrp.peers->single) {
         return iphp->get_rr_peer(pc, &iphp->rrp);
     }