Reworked IChannel.IsNsfw to support the new API flag (#771)

IChannel.IsNsfw will now return false when being used on any channel
that is not an ITextChannel. When being used on an ITextChannel, this
will now account for the API flag, and fall back to the channel name.

(this is gross design, thanks discord)

src/Discord.Net.WebSocket/Entities/Channels/SocketChannel.cs

@@ -41,7 +41,7 @@ namespace Discord.WebSocket
 
         //IChannel
         string IChannel.Name => null;
-        bool IChannel.IsNsfw => ChannelHelper.IsNsfw(this);
+        bool IChannel.IsNsfw => false;
 
         Task<IUser> IChannel.GetUserAsync(ulong id, CacheMode mode, RequestOptions options)
             => Task.FromResult<IUser>(null); //Overridden

src/Discord.Net.WebSocket/Entities/Channels/SocketTextChannel.cs

@@ -16,6 +16,7 @@ namespace Discord.WebSocket
         private readonly MessageCache _messages;
 
         public string Topic { get; private set; }
+        internal bool Nsfw { get; private set; }
 
         public string Mention => MentionUtils.MentionChannel(Id);
         public IReadOnlyCollection<SocketMessage> CachedMessages => _messages?.Messages ?? ImmutableArray.Create<SocketMessage>();
@@ -41,6 +42,7 @@ namespace Discord.WebSocket
             base.Update(state, model);
 
             Topic = model.Topic.Value;
+            Nsfw = model.Nsfw.GetValueOrDefault();
         }
 
         public Task ModifyAsync(Action<TextChannelProperties> func, RequestOptions options = null)
@@ -144,5 +146,8 @@ namespace Discord.WebSocket
             => await SendMessageAsync(text, isTTS, embed, options).ConfigureAwait(false);
         IDisposable IMessageChannel.EnterTypingState(RequestOptions options)
             => EnterTypingState(options);
+
+        // IChannel
+        bool IChannel.IsNsfw => Nsfw || ChannelHelper.IsNsfw(this);
     }
 }