Add a access modifier to the SentTime

Author: Suiram1701

WebUntisAPI.Client/Models/Messages/Message.cs

@@ -48,7 +48,7 @@ public class Message
         /// </summary>
         [JsonProperty("sentDateTime")]
         [JsonConverter(typeof(APIDateTimeJsonConverter))]
-        DateTime SentTime { get; set; }
+        public DateTime SentTime { get; set; }
 
         /// <summary>
         /// Is allowed to delete the message