Add IsNsfw to TextChannelProperties

src/Discord.Net.Rest/API/Rest/ModifyTextChannelParams.cs

@@ -8,5 +8,7 @@ namespace Discord.API.Rest
     {
         [JsonProperty("topic")]
         public Optional<string> Topic { get; set; }
+        [JsonProperty("nsfw")]
+        public Optional<bool> IsNsfw { get; set; }
     }
 }

src/Discord.Net.Rest/Entities/Channels/ChannelHelper.cs

@@ -41,7 +41,8 @@ namespace Discord.Rest
             {
                 Name = args.Name,
                 Position = args.Position,
-                Topic = args.Topic
+                Topic = args.Topic,
+                IsNsfw = args.IsNsfw
             };
             return await client.ApiClient.ModifyGuildChannelAsync(channel.Id, apiArgs, options).ConfigureAwait(false);
         }