Fix a bug where the TraceableEventDispatcher sets all event priorities to
0 when wrapping the events.

I stumpled across this problem when running multiple consecutive BrowserKit requests from a single functional test case.
Sometimes the ContextListener's kernel.response event handler was called after the TestSessionListener's handler, which leads to a started session after finishing the request.
This causes a "Cannot set session ID after the session has started." error on the subsequent request.