Add IChannel.Nsfw, RequireNsfw precondition (#633)

* Add IChannel.Nsfw, RequireNsfw precondition

* Refactored IChannel.Nsfw to IsNsfw, removed NsfwUtils

Per pull-request feedback

* proper nsfw channel check

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

@@ -1,4 +1,5 @@
-using System;
+using Discord.Rest;
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -40,6 +41,7 @@ namespace Discord.WebSocket
 
         //IChannel
         string IChannel.Name => null;
+        bool IChannel.IsNsfw => ChannelHelper.IsNsfw(this);
 
         Task<IUser> IChannel.GetUserAsync(ulong id, CacheMode mode, RequestOptions options)
             => Task.FromResult<IUser>(null); //Overridden