COMETD-430 (Allow simpler resubscriptions of dynamic subscriptions on
 re-handshake): guard against undefined subscriptions.
Fixes COMETD-424 (WebSocket HTTP headers must be case insensitive).
Fixes COMETD-428 (Enable configuration for "ws.minVersion" from
CometdServlet's init-param (web.xml)).

Parameter "ws.minVersion" is now processed by WebSocketTransport.