fix: setting firewall rules not working correctly (#405)

Because the Firewall API expects a different JSON formatting than it
responds with (some fields must be omitted even though they are included
in the response) we have to create an intermediate struct to marshal
rules properly.

See hetznercloud/cli#725

(cherry picked from commit 16daea0)

hcloud/firewall.go

@@ -298,7 +298,35 @@ type FirewallSetRulesOpts struct {
 
 // SetRules sets the rules of a Firewall.
 func (c *FirewallClient) SetRules(ctx context.Context, firewall *Firewall, opts FirewallSetRulesOpts) ([]*Action, *Response, error) {
-	reqBody := firewallSetRulesOptsToSchema(opts)
+	s := firewallSetRulesOptsToSchema(opts)
+
+	// We can't use the FirewallRule struct here, because unlike in the response some fields must be omitted when empty.
+	type firewallRule struct {
+		Direction      string   `json:"direction"`
+		SourceIPs      []string `json:"source_ips,omitempty"`
+		DestinationIPs []string `json:"destination_ips,omitempty"`
+		Protocol       string   `json:"protocol"`
+		Port           *string  `json:"port,omitempty"`
+		Description    *string  `json:"description,omitempty"`
+	}
+
+	rules := make([]firewallRule, 0, len(s.Rules))
+	for _, r := range s.Rules {
+		rules = append(rules, firewallRule{
+			Direction:      r.Direction,
+			SourceIPs:      r.SourceIPs,
+			DestinationIPs: r.DestinationIPs,
+			Protocol:       r.Protocol,
+			Port:           r.Port,
+			Description:    r.Description,
+		})
+	}
+	reqBody := struct {
+		Rules []firewallRule `json:"rules"`
+	}{
+		Rules: rules,
+	}
+
 	reqBodyData, err := json.Marshal(reqBody)
 	if err != nil {
 		return nil, nil, err