Fix typo
Fix SubProtocolHandler duplicate registration

Prior to this change, duplicate SubProtocolHandlers could be registered
when configuring STOMP with several registrations:

    public void registerStompEndpoints
          (final StompEndpointRegistry registry) {
      this.endpointRegistry.addEndpoint("/stompOverWebSocket");
      this.endpointRegistry.addEndpoint("/stompOverSockJS").withSockJS();
    }

This commit registers sub-protocols in a Set instead of a list (see
SubProtocolWebSocketHandler), thus fixing the issue.

Issue: SPR-12403
Add documentation for the keep-alive flag

This commit updates the documentation of the task:executor element to
reference the keep-alive flag.

Issue: SPR-12407
Use unversioned XSD links in code examples