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

spring-websocket/src/test/java/org/springframework/web/socket/config/annotation/WebMvcStompEndpointRegistryTests.java

@@ -84,6 +84,9 @@ public class WebMvcStompEndpointRegistryTests {
 		this.endpointRegistry.addEndpoint("/stompOverWebSocket");
 		this.endpointRegistry.addEndpoint("/stompOverSockJS").withSockJS();
 
+		//SPR-12403
+		assertEquals(1, this.webSocketHandler.getProtocolHandlers().size());
+
 		hm = (SimpleUrlHandlerMapping) this.endpointRegistry.getHandlerMapping();
 		assertEquals(2, hm.getUrlMap().size());
 		assertNotNull(hm.getUrlMap().get("/stompOverWebSocket"));