Changed Attachment.IsAppUnfurl to a nullable bool

Resolves #249

Author: soxtoby

SlackNet/Interaction/AttachmentUpdateResponse.cs

@@ -49,7 +49,7 @@ public class AttachmentUpdateResponse : IReadOnlyAttachment
     public DateTime? Timestamp => Attachment.Timestamp;
     public string ServiceName => Attachment.ServiceName;
     public string ServiceIcon => Attachment.ServiceIcon;
-    public bool IsAppUnfurl => Attachment.IsAppUnfurl;
+    public bool? IsAppUnfurl => Attachment.IsAppUnfurl;
     public string AppUnfurlUrl => Attachment.AppUnfurlUrl;
     public string BotId => Attachment.BotId;
     public string BotTeamId => Attachment.BotTeamId;

SlackNet/Objects/Attachment.cs

@@ -43,7 +43,7 @@ public interface IReadOnlyAttachment
     DateTime? Timestamp { get; }
     string ServiceName { get; }
     string ServiceIcon { get; }
-    bool IsAppUnfurl { get; }
+    bool? IsAppUnfurl { get; }
     string AppUnfurlUrl { get; }
     string BotId { get; }
     string BotTeamId { get; }
@@ -94,7 +94,7 @@ public class Attachment : IReadOnlyAttachment
     public bool? IsMsgUnfurl { get; set; }
     public string ServiceName { get; set; }
     public string ServiceIcon { get; set; }
-    public bool IsAppUnfurl { get; set; }
+    public bool? IsAppUnfurl { get; set; }
     public string AppUnfurlUrl { get; set; }
     public string BotId { get; set; }
     public string BotTeamId { get; set; }