Allow web socket client to be overridden

[activelylazy]

I'd like to be able to provide a different StandardWebSocketClient, to allow control of error behaviours and transparently re-connect the connection to slack - to do that I'd need to make client protected, so I can override in my sub-class.

[rampatra]

Thanks for the PR. Can this be done by overriding handleTransportError() method?

[activelylazy]

Oh, good question - I don't know. Not sure if I tried it, or maybe I did and it didn't work in some case. I'll try again and let you know if this PR is still required.

[activelylazy]

Ok checked the logs and I'm not seeing handleTransportError getting called.

The error I see logged looks like this:

2018-01-09 19:17:05,981 ERROR SimpleAsyncTaskExecutor-1 org.springframework.web.socket.client.WebSocketConnectionManager - Failed to connect             
javax.websocket.DeploymentException: The HTTP response from the server [503] did not permit the HTTP upgrade to WebSocket                                
        at org.apache.tomcat.websocket.WsWebSocketContainer.connectToServer(WsWebSocketContainer.java:346)                                               
        at org.springframework.web.socket.client.standard.StandardWebSocketClient$1.call(StandardWebSocketClient.java:150)                               
        at org.springframework.web.socket.client.standard.StandardWebSocketClient$1.call(StandardWebSocketClient.java:147)                               
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)         
        at java.lang.Thread.run(Thread.java:748)                            

The future returned from WebSocketClient.doHandshake sees this error (since WebSocketConnectionManager can log it). I'm thinking to create a new StandardWebSocketClient, which overrides doHandshake to register a failure callback on the returned future. I can then use that callback to trigger a re-connect.