App.filter should emit decrement when upgrading to websocket

[uNetworkingAB]

Currently you need to track App.filter events and websocket .close to get accurate connection number.

• Either App.filter should decrement when websocket.close happens (not possible)
• Or, App.filter should decrement when upgrading to websocket so that a second count for websockets can be kept from open/close events (possible).
• App.filter decrement should emit AFTER websocket upgrade so that the counter won't flicker at 0