fix(server): don't wait for socket.io store expiration timeout

lib/server.js

@@ -245,6 +245,11 @@ var createSocketIoServer = function(webServer, executor, config) {
   var server = io.listen(webServer, {
     // avoid destroying http upgrades from socket.io to get proxied websockets working
     'destroy upgrade': false,
+    // socket.io has a timeout (15s by default) before destroying a store (a data structure where
+    // data associated with a socket are stored). Unfortunately this timeout is not cleared
+    // properly on socket.io shutdown and this timeout prevents karma from exiting cleanly.
+    // We change this timeout to 0 to make Karma exit just after all tests were executed.
+    'client store expiration': 0,
     logger: logger.create('socket.io', constant.LOG_ERROR),
     resource: config.urlRoot + 'socket.io',
     transports: config.transports