Upstream: fixed "down" and "backup" parsing.

Previously arguments starting with "down" or "backup" were considered
valid, e.g. "server ... downFOO;".
diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 4a4f8eb..88d84c9 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -4701,7 +4701,7 @@
             continue;
         }
 
-        if (ngx_strncmp(value[i].data, "backup", 6) == 0) {
+        if (ngx_strcmp(value[i].data, "backup") == 0) {
 
             if (!(uscf->flags & NGX_HTTP_UPSTREAM_BACKUP)) {
                 goto invalid;
@@ -4712,7 +4712,7 @@
             continue;
         }
 
-        if (ngx_strncmp(value[i].data, "down", 4) == 0) {
+        if (ngx_strcmp(value[i].data, "down") == 0) {
 
             if (!(uscf->flags & NGX_HTTP_UPSTREAM_DOWN)) {
                 goto invalid;