Connection: prolong the AnyEvent::WebSocket::Connection's life as long as $responder to make sure it actively shuts down the socket. See the source for detail.
deleted stub t/responder.t because now it's implemented as testlib::Responder
Connection: event handlers are now kept by itself (instead of delegating to AE::WS::Conn). The callbacks are removed in DESTROY(). This design is more robust.
Add "Handlers" test. Note that with AE::WS::Conn 0.18, this test emits warnings caused by unhandled exception.