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/main/java/org/springframework/web/socket/messaging/SubProtocolWebSocketHandler.java

@@ -19,8 +19,10 @@ package org.springframework.web.socket.messaging;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import java.util.TreeMap;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicInteger;
@@ -86,7 +88,7 @@ public class SubProtocolWebSocketHandler implements WebSocketHandler,
 	private final Map<String, SubProtocolHandler> protocolHandlerLookup =
 			new TreeMap<String, SubProtocolHandler>(String.CASE_INSENSITIVE_ORDER);
 
-	private final List<SubProtocolHandler> protocolHandlers = new ArrayList<SubProtocolHandler>();
+	private final Set<SubProtocolHandler> protocolHandlers = new HashSet<SubProtocolHandler>();
 
 	private SubProtocolHandler defaultProtocolHandler;
 
@@ -129,7 +131,7 @@ public class SubProtocolWebSocketHandler implements WebSocketHandler,
 	}
 
 	public List<SubProtocolHandler> getProtocolHandlers() {
-		return new ArrayList<SubProtocolHandler>(this.protocolHandlerLookup.values());
+		return new ArrayList<SubProtocolHandler>(this.protocolHandlers);
 	}
 
 
@@ -288,7 +290,7 @@ public class SubProtocolWebSocketHandler implements WebSocketHandler,
 				handler = this.defaultProtocolHandler;
 			}
 			else if (this.protocolHandlers.size() == 1) {
-					handler = this.protocolHandlers.get(0);
+				handler = this.protocolHandlers.iterator().next();
 			}
 			else {
 				throw new IllegalStateException("Multiple protocol handlers configured and " +

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"));