Update OneSignalClient.php

add UTF-8 encoding

Author: Hussam3bd

src/OneSignalClient.php

@@ -141,7 +141,7 @@ public function sendNotificationUsingTags(
         $schedule = null
     ) {
         $contents = [
-            "en" => $message
+            'en' => mb_convert_encoding($message, 'UTF-8', 'UTF-8')
         ];
 
         $params = [
@@ -437,4 +437,4 @@ public function put($endPoint)
 
         return $this->client->put(self::API_URL . $endPoint, $this->headers);
     }
-}
+}