Fix SocketCategoryChannel properties (discord-net#945)

* Update Users property for category channels

* Wrong property being used for Channels property

CategoryId is the category that "owns" this channel. That is actually impossible right now for category channels, so it returns null and get all channels wrongly.

* Resolve permissions for category

* Remove spaces

* Small fix for IChannel.GetUsersAsync

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

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
@@ -15,10 +15,12 @@ namespace Discord.WebSocket
     public class SocketCategoryChannel : SocketGuildChannel, ICategoryChannel
     {
         public override IReadOnlyCollection<SocketGuildUser> Users
-            => Guild.Users.Where(x => x.VoiceChannel?.Id == Id).ToImmutableArray();
+            => Guild.Users.Where(x => Permissions.GetValue(
+               Permissions.ResolveChannel(Guild, x, this, Permissions.ResolveGuild(Guild, x)),
+               ChannelPermission.ViewChannel)).ToImmutableArray();
 
         public IReadOnlyCollection<SocketGuildChannel> Channels
-            => Guild.Channels.Where(x => x.CategoryId == CategoryId).ToImmutableArray();
+            => Guild.Channels.Where(x => x.CategoryId == Id).ToImmutableArray();
 
         internal SocketCategoryChannel(DiscordSocketClient discord, ulong id, SocketGuild guild)
             : base(discord, id, guild)
@@ -31,23 +33,37 @@ internal new static SocketCategoryChannel Create(SocketGuild guild, ClientState
             return entity;
         }
 
+        //Users
+        public override SocketGuildUser GetUser(ulong id)
+        {
+            var user = Guild.GetUser(id);
+            if (user != null)
+            {
+                var guildPerms = Permissions.ResolveGuild(Guild, user);
+                var channelPerms = Permissions.ResolveChannel(Guild, user, this, guildPerms);
+                if (Permissions.GetValue(channelPerms, ChannelPermission.ViewChannel))
+                    return user;
+            }
+            return null;
+        }
+
         private string DebuggerDisplay => $"{Name} ({Id}, Category)";
         internal new SocketCategoryChannel Clone() => MemberwiseClone() as SocketCategoryChannel;
 
         // IGuildChannel
         IAsyncEnumerable<IReadOnlyCollection<IGuildUser>> IGuildChannel.GetUsersAsync(CacheMode mode, RequestOptions options)
-            => throw new NotSupportedException();
+            => ImmutableArray.Create<IReadOnlyCollection<IGuildUser>>(Users).ToAsyncEnumerable();
         Task<IGuildUser> IGuildChannel.GetUserAsync(ulong id, CacheMode mode, RequestOptions options)
-            => throw new NotSupportedException();
+            => Task.FromResult<IGuildUser>(GetUser(id));
         Task<IInviteMetadata> IGuildChannel.CreateInviteAsync(int? maxAge, int? maxUses, bool isTemporary, bool isUnique, RequestOptions options)
             => throw new NotSupportedException();
         Task<IReadOnlyCollection<IInviteMetadata>> IGuildChannel.GetInvitesAsync(RequestOptions options)
             => throw new NotSupportedException();
 
         //IChannel
         IAsyncEnumerable<IReadOnlyCollection<IUser>> IChannel.GetUsersAsync(CacheMode mode, RequestOptions options)
-            => throw new NotSupportedException();
+            => ImmutableArray.Create<IReadOnlyCollection<IUser>>(Users).ToAsyncEnumerable();
         Task<IUser> IChannel.GetUserAsync(ulong id, CacheMode mode, RequestOptions options)
-            => throw new NotSupportedException();
+            => Task.FromResult<IUser>(GetUser(id));
     }
 }