fix: 🐛 webpush messages were not UTF-8 encoded

Webpush messages was encoded using the default encoding, thus generating
errors when sending UTF-8 texts

push-sender/src/main/java/org/jboss/aerogear/unifiedpush/message/webpush/WebPushSender.java

@@ -100,7 +100,7 @@ public void sendPushMessage(final Variant variant, final Collection<String> toke
 
                 try {
                     final Notification notification = new Notification(registration.getEndpoint(), getUserPublicKey(registration),
-                            registration.getAuthAsBytes(), gson.toJson(pushMessage.getMessage()).getBytes());
+                            registration.getAuthAsBytes(), gson.toJson(pushMessage.getMessage()).getBytes("UTF-8"));
 
                     final HttpResponse response = webPushService.send(notification);
                     final int responseCode = response.getStatusLine().getStatusCode();