don't put extension params in 'CreateOpts' if unset; working rackspace acceptance test
diff --git a/openstack/compute/v2/extensions/diskconfig/requests.go b/openstack/compute/v2/extensions/diskconfig/requests.go
index cc390f7..7407e0d 100644
--- a/openstack/compute/v2/extensions/diskconfig/requests.go
+++ b/openstack/compute/v2/extensions/diskconfig/requests.go
@@ -41,7 +41,7 @@
 	servers.CreateOptsBuilder
 
 	// DiskConfig [optional] controls how the created server's disk is partitioned.
-	DiskConfig DiskConfig
+	DiskConfig DiskConfig `json:"OS-DCF:diskConfig,omitempty"`
 }
 
 // ToServerCreateMap adds the diskconfig option to the base server creation options.
@@ -51,6 +51,10 @@
 		return nil, err
 	}
 
+	if string(opts.DiskConfig) == "" {
+		return base, nil
+	}
+
 	serverMap := base["server"].(map[string]interface{})
 	serverMap["OS-DCF:diskConfig"] = string(opts.DiskConfig)