fix nil option in unit test
diff --git a/openstack/objectstorage/v1/containers/requests_test.go b/openstack/objectstorage/v1/containers/requests_test.go
index d58ebeb..0f1a9a1 100644
--- a/openstack/objectstorage/v1/containers/requests_test.go
+++ b/openstack/objectstorage/v1/containers/requests_test.go
@@ -32,7 +32,7 @@
testhelper.TestHeader(t, r, "X-Auth-Token", tokenId)
testhelper.TestHeader(t, r, "Accept", "application/json")
- w.Header().Add("Content-Type", "application/json")
+ w.Header().Set("Content-Type", "application/json")
r.ParseForm()
marker := r.Form.Get("marker")
switch marker {