dont omit nodebalancers on create firewall

firewalls.go

@@ -33,7 +33,7 @@ type Firewall struct {
 // DevicesCreationOptions fields are used when adding devices during the Firewall creation process.
 type DevicesCreationOptions struct {
 	Linodes       []int `json:"linodes,omitempty"`
-	NodeBalancers []int `json:"-"`
+	NodeBalancers []int `json:"nodebalancers,omitempty"`
 }
 
 // FirewallCreateOptions fields are those accepted by CreateFirewall