Allow modifying HandshakeInterceptor list

The getter in TransportHandlingSockJsService now returns a mutable
List. The immutable wrapper doesn't make sense since it's possible
anyway to modify the list by creating a new list and calling the
setter again. It's also consistent with the same field on
WebSocketHttpRequestHandler.

This is related to work for SPR-12845.

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/TransportHandlingSockJsService.java

@@ -146,7 +146,7 @@ public class TransportHandlingSockJsService extends AbstractSockJsService implem
 	 * Return the configured WebSocket handshake request interceptors.
 	 */
 	public List<HandshakeInterceptor> getHandshakeInterceptors() {
-		return Collections.unmodifiableList(this.interceptors);
+		return this.interceptors;
 	}