Modules: fixed Respose.headers getter in fetch API.

The issue manifested itself when Response object is dumped using
JSON.stringify() or njs.dump().  The Response headers were dumped
as "null" values.
diff --git a/nginx/ngx_js_fetch.c b/nginx/ngx_js_fetch.c
index 4795c87..c88f590 100644
--- a/nginx/ngx_js_fetch.c
+++ b/nginx/ngx_js_fetch.c
@@ -2207,8 +2207,7 @@
         return NJS_ERROR;
     }
 
-    return ngx_response_js_ext_header_get(vm, value, &name, njs_vm_retval(vm),
-                                          0);
+    return ngx_response_js_ext_header_get(vm, value, &name, retval, 0);
 }