DRY webhook signing key call

mailgun.go

@@ -345,7 +345,11 @@ func (mg *MailgunImpl) SetClient(c *http.Client) {
 
 // WebhookSigningKey returns the webhook signing key configured for this client
 func (mg *MailgunImpl) WebhookSigningKey() string {
-	return mg.webhookSigningKey
+	key := mg.webhookSigningKey
+	if key == "" {
+		return mg.APIKey()
+	}
+	return key
 }
 
 // SetWebhookSigningKey updates the webhook signing key for this client

webhooks.go

@@ -123,12 +123,7 @@ type WebhookPayload struct {
 
 // Use this method to parse the webhook signature given as JSON in the webhook response
 func (mg *MailgunImpl) VerifyWebhookSignature(sig Signature) (verified bool, err error) {
-	key := mg.WebhookSigningKey()
-	// For backwards compatibility, fall back to the api key
-	if key == "" {
-		key = mg.APIKey()
-	}
-	h := hmac.New(sha256.New, []byte(key))
+	h := hmac.New(sha256.New, []byte(mg.WebhookSigningKey()))
 
 	_, err = io.WriteString(h, sig.TimeStamp)
 	if err != nil {
@@ -154,12 +149,7 @@ func (mg *MailgunImpl) VerifyWebhookSignature(sig Signature) (verified bool, err
 // Deprecated: Please use the VerifyWebhookSignature() to parse the latest
 // version of WebHooks from mailgun
 func (mg *MailgunImpl) VerifyWebhookRequest(req *http.Request) (verified bool, err error) {
-	key := mg.WebhookSigningKey()
-	// For backwards compatibility, fall back to the api key
-	if key == "" {
-		key = mg.APIKey()
-	}
-	h := hmac.New(sha256.New, []byte(key))
+	h := hmac.New(sha256.New, []byte(mg.WebhookSigningKey()))
 
 	_, err = io.WriteString(h, req.FormValue("timestamp"))
 	if err != nil {