Added missing registration_id field to FcmResult

Author: danielgindi

CorePush/Google/FcmResult.cs

@@ -7,6 +7,9 @@ public class FcmResult
         [JsonProperty("message_id")]
         public string MessageId { get; set; }
 
+        [JsonProperty("registration_id")]
+        public string RegistrationId { get; set; }
+
         public string Error { get; set; }
     }
 }