fix: handle disposed socket on close attempt

[ezhevita]

Fixes #524.

Some investigation

Originally I had the following stacktrace for ArchiSteamFarm:

System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'System.Net.WebSockets.ClientWebSocket'.
   at System.Net.WebSockets.ClientWebSocket.get_ConnectedWebSocket()
   at SteamKit2.WebSocketConnection.WebSocketContext.RunCore(HttpMessageInvoker invoker, TimeSpan connectionTimeout, CancellationToken cancellationToken)
   at SteamKit2.TaskExtensions.IgnoringCancellation(Task task, CancellationToken token)

ConnectedWebSocket is a private property and there is no mention of it in SteamKit codebase whatsoever, so there is definitely some method that got inlined and its stackframe was omitted.
ClientWebSocket accesses that property in 7 methods:

• SendAsync (+2 overloads),
• ReceiveAsync (+1 overload),
• CloseAsync,
• CloseOutputAsync.

WebSocketContext calls these methods in these cases:

• SendAsync is called from WebSocketContext.SendAsync <- WebSocketConnection.Send, which doesn't match our stacktrace,
• ReceiveAsync is called from ReadMessageAsync, but the ObjectDisposedException is explicitly handled there,
• CloseAsync seems to be the only suitable candidate.

[JustArchi]

Should be sufficient, you don't need to release semaphore to dispose it.