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.Rest/API/Common/Channel.cs

@@ -29,6 +29,8 @@ namespace Discord.API
         public Optional<string> Topic { get; set; }
         [JsonProperty("last_pin_timestamp")]
         public Optional<DateTimeOffset?> LastPinTimestamp { get; set; }
+        [JsonProperty("nsfw")]
+        public Optional<bool> Nsfw { get; set; }
 
         //VoiceChannel
         [JsonProperty("bitrate")]

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

@@ -290,8 +290,8 @@ namespace Discord.Rest
             return author;
         }
 
-        public static bool IsNsfw(IChannel channel) =>
-            IsNsfw(channel.Name);
+        public static bool IsNsfw(IChannel channel)
+            => IsNsfw(channel.Name);
         public static bool IsNsfw(string channelName) =>
             channelName == "nsfw" || channelName.StartsWith("nsfw-");
     }

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

@@ -46,7 +46,7 @@ namespace Discord.Rest
 
         //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); //Overriden

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

@@ -15,6 +15,8 @@ namespace Discord.Rest
 
         public string Mention => MentionUtils.MentionChannel(Id);
 
+        internal bool Nsfw { get; private set; }
+
         internal RestTextChannel(BaseDiscordClient discord, IGuild guild, ulong id)
             : base(discord, guild, id)
         {
@@ -30,6 +32,7 @@ namespace Discord.Rest
             base.Update(model);
 
             Topic = model.Topic.Value;
+            Nsfw = model.Nsfw.GetValueOrDefault();
         }
 
         public async Task ModifyAsync(Action<TextChannelProperties> func, RequestOptions options = null)
@@ -149,5 +152,6 @@ namespace Discord.Rest
             else
                 return AsyncEnumerable.Empty<IReadOnlyCollection<IGuildUser>>();
         }
+        bool IChannel.IsNsfw => Nsfw || ChannelHelper.IsNsfw(this);
     }
 }