SocketSelfUser inherits ISelfUser

Resolves #159.

The previous bug was that SocketSelfUser did not inherit from ISelfUser, so DiscordSocketClient.GetCurrentUserAsync was not returning a SocketSelfUser, despite the underlying type being that. This caused ModifyStatusAsync to throw the NotSupportedException that existed on the REST SelfUser.

src/Discord.Net/WebSocket/Entities/Users/SocketSelfUser.cs

@@ -6,7 +6,7 @@ using Model = Discord.API.User;
 
 namespace Discord
 {
-    internal class SocketSelfUser : SelfUser, ISocketUser
+    internal class SocketSelfUser : SelfUser, ISocketUser, ISelfUser
     {
         internal override bool IsAttached => true;