[fix] Do not unconditionally set the ready state to -2

If we're already closing, then we shouldn't move into the new -2 ready
state (i.e. closing-because-the-remote-end-closed). We're already
closing, we shouldn't go back to a state that allows sending another
close frame.

Author: pimterry

lib/websocket.js

@@ -940,7 +940,9 @@ function socketOnEnd() {
   // the closing handshake but unlike `WebSocket.CLOSING` it does not prevent
   // `WebSocket.prototype.close()` from sending a close frame.
   //
-  websocket._readyState = -2;
+  if (websocket._readyState === WebSocket.OPEN) {
+    websocket._readyState = -2;
+  }
   websocket._receiver.end();
 }