Unify ShardedCommandContext with SocketCommandContext (#739)

* Make ShardedCommandContext derive from SocketCommandContext

* Explicitly re-implement ICommandContext.Client

src/Discord.Net.WebSocket/Commands/ShardedCommandContext.cs

@@ -2,30 +2,20 @@
 
 namespace Discord.Commands
 {
-    public class ShardedCommandContext : ICommandContext
+    public class ShardedCommandContext : SocketCommandContext, ICommandContext
     {
-        public DiscordShardedClient Client { get; }
-        public SocketGuild Guild { get; }
-        public ISocketMessageChannel Channel { get; }
-        public SocketUser User { get; }
-        public SocketUserMessage Message { get; }
-
-        public bool IsPrivate => Channel is IPrivateChannel;
+        public new DiscordShardedClient Client { get; }
 
         public ShardedCommandContext(DiscordShardedClient client, SocketUserMessage msg)
+            : base(client.GetShard(GetShardId(client, (msg.Channel as SocketGuildChannel)?.Guild)), msg)
         {
             Client = client;
-            Guild = (msg.Channel as SocketGuildChannel)?.Guild;
-            Channel = msg.Channel;
-            User = msg.Author;
-            Message = msg;
         }
 
+        private static int GetShardId(DiscordShardedClient client, IGuild guild)
+            => guild == null ? 0 : client.GetShardIdFor(guild);
+
         //ICommandContext
         IDiscordClient ICommandContext.Client => Client;
-        IGuild ICommandContext.Guild => Guild;
-        IMessageChannel ICommandContext.Channel => Channel;
-        IUser ICommandContext.User => User;
-        IUserMessage ICommandContext.Message => Message;
     }
 }