fix ListByServer URL and unit test
diff --git a/openstack/compute/v2/extensions/secgroups/urls.go b/openstack/compute/v2/extensions/secgroups/urls.go
index f4760b6..dc53fbf 100644
--- a/openstack/compute/v2/extensions/secgroups/urls.go
+++ b/openstack/compute/v2/extensions/secgroups/urls.go
@@ -16,7 +16,7 @@
 }
 
 func listByServerURL(c *gophercloud.ServiceClient, serverID string) string {
-	return c.ServiceURL(secgrouppath, "servers", serverID, secgrouppath)
+	return c.ServiceURL("servers", serverID, secgrouppath)
 }
 
 func rootRuleURL(c *gophercloud.ServiceClient) string {