[websocket] Implement HttpListenerContext.AcceptWebSocketAsync().
[websocket] Cleaner closing.
[websocket] Better hijacking.
[websocket] Use string.Equals instead of String.Compare
[websocket] Don't use string +=
Reimplement StreamWebSocket.
Fix some bugs.
Support CloseStatus
Support receiving frame larger than buffer.
Use do ... while (true)
Fix handling fragment and opcode.
Fix indent of author.
Remove checking Origin header.

RFC requires Origin header for browser client.
Non browser client may be omit Origin header.
Merge pull request #981 from methane/websocket

[websocket] Implement HttpListenerContext.AcceptWebSocketAsync()