Allow return code 200 when adding role to user (#40)

diff --git a/openstack/identity/v2/extensions/admin/roles/requests.go b/openstack/identity/v2/extensions/admin/roles/requests.go
index 4d27972..50228c9 100644
--- a/openstack/identity/v2/extensions/admin/roles/requests.go
+++ b/openstack/identity/v2/extensions/admin/roles/requests.go
@@ -17,7 +17,9 @@
 // a user. This is confined to the scope of the user's tenant - so the tenant
 // ID is a required argument.
 func AddUser(client *gophercloud.ServiceClient, tenantID, userID, roleID string) (r UserRoleResult) {
-	_, r.Err = client.Put(userRoleURL(client, tenantID, userID, roleID), nil, nil, nil)
+	_, r.Err = client.Put(userRoleURL(client, tenantID, userID, roleID), nil, nil, &gophercloud.RequestOpts{
+		OkCodes: []int{200, 201},
+	})
 	return
 }