add union auth request handler

@liggitt I chose not to change behavior, but I don't see a reason why AuthenticationSuccessHandlers should be the exact same object as AuthenticationRequestHandlers.  If you agree that such a restriction is unnecessary, I'll turn getAuthenticationSuccessHandlers into a method that checks the configuration and creates the necessary handlers based on knowledge of both AuthenticationRequestHandler config and AuthenticationHandler config.
Maybe it was just the two flags that bothered me, one to tell you you have multiple auth handlers and the other telling you the multiple auth handlers. Not a big deal