Fix minor issue in WebSocketHttpRequestHandler

Issue: SPR-10721
Cache constructor instance in WebAsyncUtils

Issue: SPR-10673
Enable asyncSupported flag in Spring Test MVC

Also remove Servlet 3 bridge classes no longer needed since Spring
Framework 4 compiles with the Servlet 3 API. That means applications
may need to run tests with the Servlet 3 API jar although
technically they don't have to run with that in production.